* Handle when there are multiple workspaces migrating to cloud,
using both the cloud name strategy and cloud tags strategy.
* Add e2e tests

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.

Run variables allow the run API to accept input variables not found
in the configuration slug (the file-based ones plus workspace vars)

1. ParseDeclaredValues: parses unparsed variables into terraform.InputValues
2. ProbeUndeclaredVariableValues: compares variable declarations with unparsed values to warn/error about undeclared variables

* determining source or destination to cloud
* handling single to single state migrations to cloud,
using a name strategy or a tags strategy
* Add end-to-end tests for state migration.

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.

This is a port of the changes made in #29683

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.

Implementing this test was quite a rabbithole, as in order to satisfy
backendTestBackendStates() the workspaces returned from
backend.Workspaces() must match exactly, and the shortcut taken to test
pagination in 3cc58813 created an
impossible circumstance that got plastered over with the fact that
prefix filtering is done clientside, not by the API as it should be.

Tagging does not rely on clientside filtering, and expects that the
request made to the TFC API returns exactly those workspaces with the
given tags.

These changes include a better way to test pagination, wherein we
actually create over a page worth of valid workspaces in the mock client
and implement a simplified pagination behavior to match how the TFC API
actually works.

The TFC API doesn't behave this way; a search term just signals the
substring was found within the name.

"NoDefault" is now ambiguous with tags, and it does not imply using a
prefix.

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

These changes include additions to fulfill the interface for the client
mock, plus moving all that logic (which needn't be duplicated across
both the remote and cloud packages) over to the cloud package under a
dedicated mock client file.

I missed adding these in the original porting commit from the remote
backend, because *.log is added to gitignore.