This can help differentiate cloud integration API requests from normal remote backend requests

When the 'select the exact version if possible' behavior was added, the
version check below it was never updated to take the newly updated
version in to account, resulting in a failed version check even as the
remote workspace updated to the correct version necessary.

Previously, if the remote TFC/TFE instance doesn't happen to have a tool_version
record whose name exactly matches the value of `tfversion.String()`, Terraform
would be completely blocked from using the `terraform workspace new` command
(when configured with the tags strategy) — the API would give a 422 to the
whole create request.

This commit changes the StateMgr() function to do the work in two passes; first
create the workspace (which should work fine regardless), THEN update the
Terraform version and print a warning to the terminal if it fails (which 99% of
the time is a benign failure with little impact on your future CLI usage).

Explicit version strings are actually also version constraints! And the special
comparisons we were doing to allow a range of compatible versions can also be
expressed as version constraints.

Bonus: also simplify the way we handle version check errors, by composing the
messages inline and only extracting the repetitive parts into a function.

The cloud backend (and remote before it) previously expected a TFC workspace's
`terraform-version` attribute to be either the magic string `"latest"` or an
explicit semver value. But a workspace might have a version constraint instead
(like `~> 1.1.0`), in which case the version check would blow up.

This commit checks whether `terraform-version` is a valid version constraint
before erroring out, and if so, returns success if the local version meets the
constraint.

Because it's not practical to deeply introspect the slice of version space
defined by a constraint, this check is slightly less robust than the version
comparisons below it:

- It can give a false OK on open-ended constraints like `>= 1.1.0`. Say you're
  running 1.3.0, it changed the state format, and the TFE instance admin has
  not yet added any 1.3.x Terraform versions; your workspace will now break.

- It will give a false not-OK when using different minor versions within a range
  that we know to be compatible, e.g. remote constraint of `~> 0.15.0` and local
  version of 1.1.0.

- This would be totally useless with the pre-0.14 versions of Terraform, where
  patch releases could change state format... but we're not going back in time
  to add this feature to them anyway.

Still, in the most common likely case (`~> x.y.z`), it'll complain at you (with
an error you can choose to override) if you're not using the same minor version,
and that seems proportionate, useful, and expected.

Now that we have tags we no longer need prefix.

When a user runs `terraform refresh` we give them an error message that
tells them to run `terraform apply -refresh-state`. We could just run
that command for them, though. That is what this PR does.

These changes remove all of the preexisting version checking for
individual features, wiping the slate clean with an overall minimum
requirement of a future TFP-API-Version 2.5, which at the time of this
writing is expected to be TFE v202112-1.

It also actually provides that expected TFE version as an actionable
error message, rather than generically saying that it isn't supported or
using the somewhat opaque API version header.

The 'tfe' service was appended to with various versions to denote a new
'feature' implemented by a new 'service'. This quickly proved to not be
scalable, as adding an entry to the discovery document from every
feature is bad.

The new mechanism added was checking the TFP-API-Version header on
requests for a version, instead.

So we'll remove the separation here between different tfe service
'versions' and the separate 'state' service and Just Use TFE, as well as
the TFP-API-Version header for all feature versioning., as well as the
TFP-API-Version header for all feature versioning.

This is an outdated mechanism that isn't used anymore.

The previous conservative guarantee that we would not make backwards
incompatible changes to the state file format until at least Terraform
1.1 can now be extended. Terraform 0.14 through 1.1 will be able to
interoperably use state files, so we can update the remote backend
version compatibility check accordingly.

This is a port of https://github.com/hashicorp/terraform/pull/29645

This changes the 'name' strategy to always align the local configured
workspace name and the remote Terraform Cloud workspace, rather than the
implicit use of the 'default' unnamed workspace being used instead.

What this essentially means is that the Cloud integration does not fully
support workspaces when configured for a single TFC workspace (as was
the case with the 'remote' backend), but *does* use the
backend.Workspaces() interface to allow for normal local behaviors like
terraform.workspace to resolve to the correct name. It does this by
always setting the local workspace name when the 'name' strategy is
used, as a part of initialization.

Part of the diff here is exporting all the previously unexported types
for mapping workspaces. The command package (and init in particular)
needs to be able to handle setting the local workspace in this
particular scenario.

A mostly cosemetic change; The fields 'workspace' and 'prefix' don't
really describe well what they are from a caller, so change these to use
a workspaceMapping struct to convey they are for implementing workspace
mapping strategies from CLI -> TFC

This makes the new backend compatible with the new terraform.Context API,
which has changed in main.

The cloud package intends to implement a new integration for
Terraform Cloud/Enterprise. The purpose of this integration is to better
support TFC users; it will shed some overly generic UX and architecture,
behavior changes that are otherwise backwards incompatible in the remote
backend, and technical debt - all of which are vestiges from before
Terraform Cloud existed.

This initial commit is largely a porting of the existing 'remote'
backend, which will serve as an underlying implementation detail and not
be a typical user-level backend. This is because to re-implement the
literal backend interface is orthogonal to the purpose of this
integration, and can always be migrated away from later.

As this backend is considered an implementation detail, it will not be
registered as a declarable backend. Within these changes it is, for easy
of initial development and a clean diff.

In historical versions of Terraform the responsibility to check this was
inside the terraform.NewContext function, along with various other
assorted concerns that made that function particularly complicated.

More recently, we reduced the responsibility of the "terraform" package
only to instantiating particular named plugins, assuming that its caller
is responsible for selecting appropriate versions of any providers that
_are_ external. However, until this commit we were just assuming that
"terraform init" had correctly selected appropriate plugins and recorded
them in the lock file, and so nothing was dealing with the problem of
ensuring that there haven't been any changes to the lock file or config
since the most recent "terraform init" which would cause us to need to
re-evaluate those decisions.

Part of the game here is to slightly extend the role of the dependency
locks object to also carry information about a subset of provider
addresses whose lock entries we're intentionally disregarding as part of
the various little edge-case features we have for overridding providers:
dev_overrides, "unmanaged providers", and the testing overrides in our
own unit tests. This is an in-memory-only annotation, never included in
the serialized plan files on disk.

I had originally intended to create a new package to encapsulate all of
this plugin-selection logic, including both the version constraint
checking here and also the handling of the provider factory functions, but
as an interim step I've just made version constraint consistency checks
the responsibility of the backend/local package, which means that we'll
always catch problems as part of preparing for local operations, while
not imposing these additional checks on commands that _don't_ run local
operations, such as "terraform apply" when in remote operations mode.

The previous conservative guarantee that we would not make backwards
incompatible changes to the state file format until at least Terraform
1.1 can now be extended. Terraform 0.14 through 1.1 will be able to
interoperably use state files, so we can update the remote backend
version compatibility check accordingly.

Previously terraform.Context was built in an unfortunate way where all of
the data was provided up front in terraform.NewContext and then mutated
directly by subsequent operations. That made the data flow hard to follow,
commonly leading to bugs, and also meant that we were forced to take
various actions too early in terraform.NewContext, rather than waiting
until a more appropriate time during an operation.

This (enormous) commit changes terraform.Context so that its fields are
broadly just unchanging data about the execution context (current
workspace name, available plugins, etc) whereas the main data Terraform
works with arrives via individual method arguments and is returned in
return values.

Specifically, this means that terraform.Context no longer "has-a" config,
state, and "planned changes", instead holding on to those only temporarily
during an operation. The caller is responsible for propagating the outcome
of one step into the next step so that the data flow between operations is
actually visible.

However, since that's a change to the main entry points in the "terraform"
package, this commit also touches every file in the codebase which
interacted with those APIs. Most of the noise here is in updating tests
to take the same actions using the new API style, but this also affects
the main-code callers in the backends and in the command package.

My goal here was to refactor without changing observable behavior, but in
practice there are a couple externally-visible behavior variations here
that seemed okay in service of the broader goal:
 - The "terraform graph" command is no longer hooked directly into the
   core graph builders, because that's no longer part of the public API.
   However, I did include a couple new Context functions whose contract
   is to produce a UI-oriented graph, and _for now_ those continue to
   return the physical graph we use for those operations. There's no
   exported API for generating the "validate" and "eval" graphs, because
   neither is particularly interesting in its own right, and so
   "terraform graph" no longer supports those graph types.
 - terraform.NewContext no longer has the responsibility for collecting
   all of the provider schemas up front. Instead, we wait until we need
   them. However, that means that some of our error messages now have a
   slightly different shape due to unwinding through a differently-shaped
   call stack. As of this commit we also end up reloading the schemas
   multiple times in some cases, which is functionally acceptable but
   likely represents a performance regression. I intend to rework this to
   use caching, but I'm saving that for a later commit because this one is
   big enough already.

The proximal reason for this change is to resolve the chicken/egg problem
whereby there was previously no single point where we could apply "moved"
statements to the previous run state before creating a plan. With this
change in place, we can now do that as part of Context.Plan, prior to
forking the input state into the three separate state artifacts we use
during planning.

However, this is at least the third project in a row where the previous
API design led to piling more functionality into terraform.NewContext and
then working around the incorrect order of operations that produces, so
I intend that by paying the cost/risk of this large diff now we can in
turn reduce the cost/risk of future projects that relate to our main
workflow actions.

This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.

If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.

This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.

If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.

This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.

If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.

This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.

If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.

This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.

If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.

When we fail to read the organization entitlements, it's not
always because the organization doesn't exist; in practice, it's
usually due to a misspelled organization name, forgetting to
specify the correct hostname, or an invalid API token. Surfacing
more information within the error message will help new users
debug these issues more effectively.

To ensure that the apply command can determine whether an operation is
executed locally or remotely, we add an IsLocalOperations method on the
remote backend. This returns the internal forceLocal boolean.

We also update this flag after checking if the corresponding remote
workspace is in local operations mode or not. This ensures that we know
if an operation is running locally (entirely on the practitioner's
machine), pseudo-locally (on a Terraform Cloud worker), or remotely
(executing on a worker, rendering locally).

When migrating state to a new workspace, the version check would error
due to a 404 error on fetching the workspace record. This would result
in failed state migration.

Instead we should look specifically for a 404 error, and allow migration
to continue. If we're just about to create the workspace, there can't be
a version incompatibility problem.

The enhanced backends (local and remote) need to be able to render
diagnostics during operations. Prior to this commit, this functionality
was supported with a per-backend `ShowDiagnostics` function pointer.

In order to allow users of these backends to control how diagnostics are
rendered, this commit moves that function pointer to the `Operation`
type. This means that a diagnostic renderer is configured for each
operation, rather than once per backend initialization.

Some secondary consequences of this change:

- The `ReportResult` method on the backend is now moved to the
  `Operation` type, as it needs to access the `ShowDiagnostics` callback
  (and nothing else from the backend);
- Tests which assumed that diagnostics would be written to the backend's
  `cli.Ui` instance are migrated to using a new record/playback diags
  helper function;
- Apply, plan, and refresh commands now pass a pointer to the `Meta`
  struct's `showDiagnostics` method.

This com...

If the remote backend is connected to a Terraform Cloud workspace in
local operations mode, we disable the version check, as the remote
Terraform version is meaningless.