We originally introduced the idea of language experiments as a way to get
early feedback on not-yet-proven feature ideas, ideally as part of the
initial exploration of the solution space rather than only after a
solution has become relatively clear.

Unfortunately, our tradeoff of making them available in normal releases
behind an explicit opt-in in order to make it easier to participate in the
feedback process had the unintended side-effect of making it feel okay
to use experiments in production and endure the warnings they generate.
This in turn has made us reluctant to make use of the experiments feature
lest experiments become de-facto production features which we then feel
compelled to preserve even though we aren't yet ready to graduate them
to stable features.

In an attempt to tweak that compromise, here we make the availability of
experiments _at all_ a build-time flag which will not be set by default,
and therefore experiments will not be available ...

Error diags from c.installModules() no longer cause getModules() to exit early.
Whether installModules completed successfully, errored, or was cancelled, we
try to update the manifest as best we can, preferring incomplete information
to none.

Earlier work to make "terraform init" interruptible made the getproviders
package context-aware in order to allow provider installation to be cancelled.

Here we make a similar change for module installation, which is now also
cancellable with SIGINT. This involves plumbing context through initwd and
getmodules. Functions which can make network requests now include a context
parameter whose cancellation cancels those requests.

Since the module installation code is shared, "terraform get" is now
also interruptible during module installation.

This is a replacement declaration for using Terraform Cloud as a remote
backend, leaving the literal backend as an implementation detail and not
a user-level concept.

Thus far our various interactions with the bits of state we keep
associated with a working directory have all been implemented directly
inside the "command" package -- often in the huge command.Meta type -- and
not managed collectively via a single component.

There's too many little codepaths reading and writing from the working
directory and data directory to refactor it all in one step, but this is
an attempt at a first step towards a future where everything that reads
and writes from the current working directory would do so via an object
that encapsulates the implementation details and offers a high-level API
to read and write all of these session-persistent settings.

The design here continues our gradual path towards using a dependency
injection style where "package main" is solely responsible for directly
interacting with the OS command line, the OS environment, the OS working
directory, the stdio streams, and the CLI configura...

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.

Terraform supports multiple output formats for several sub-commands.
The default format is user-readable text, but many sub-commands support
a `-json` flag to output a machine-readable format for the result. The
output command also supports a `-raw` flag for a simpler, scripting-
focused machine readable format.

This commit adds a "views" abstraction, intended to help ensure
consistency between the various output formats. This extracts the render
specific code from the command package, and moves it into a views
package. Each command is expected to create an interface for its view,
and one or more implementations of that interface.

By doing so, we separate the concerns of generating the sub-command
result from rendering the result in the specified output format. This
should make it easier to ensure that all output formats will be updated
together when changes occur in the result-generating phase.

There are some other consequences of this restructuring:

- Views now directly access the terminal streams, rather than the
  now-redundant cli.Ui instance;
- With the reorganization of commands, parsing CLI arguments is now the
  responsibility of a separate "arguments" package.

For now, views are added only for the output sub-command, as an example.
Because this command uses code which is shared with the apply and
refresh commands, those are also partially updated.

* remove unused code

I've removed the provider-specific code under registry, and unused nil
backend, and replaced a call to helper from backend/oss (the other
callers of that func are provisioners scheduled to be deprecated).

I also removed the Dockerfile, as our build process uses a different
file.

Finally I removed the examples directory, which had outdated examples
and links. There are better, actively maintained examples available.

* command: remove various unused bits

* test wasn't running

* backend: remove unused err

Any warnings will be caught again when the entire config is loaded, and
duplicated in the output.

Previously we were using the experimental HCL 2 repository, but now we'll
shift over to the v2 import path within the main HCL repository as part of
actually releasing HCL 2.0 as stable.

This is a mechanical search/replace to the new import paths. It also
switches to the v2.0.0 release of HCL, which includes some new code that
Terraform didn't previously have but should not change any behavior that
matters for Terraform's purposes.

For the moment the experimental HCL2 repository is still an indirect
dependency via terraform-config-inspect, so it remains in our go.sum and
vendor directories for the moment. Because terraform-config-inspect uses
a much smaller subset of the HCL2 functionality, this does still manage
to prune the vendor directory a little. A subsequent release of
terraform-config-inspect should allow us to completely remove that old
repository in a future commit.

We can't dereference the config module if there were any errors loading
it.

There are a few constructs from 0.11 and prior that cause 0.12 parsing to
fail altogether, which previously created a chicken/egg problem because
we need to install the providers in order to run "terraform 0.12upgrade"
and thus fix the problem.

This changes "terraform init" to use the new "early configuration" loader
for module and provider installation. This is built on the more permissive
parser in the terraform-config-inspect package, and so it allows us to
read out the top-level blocks from the configuration while accepting
legacy HCL syntax.

In the long run this will let us do version compatibility detection before
attempting a "real" config load, giving us better error messages for any
future syntax additions, but in the short term the key thing is that it
allows us to install the dependencies even if the configuration isn't
fully valid.

Because backend init still requires full configuration, this introduces a
new mode of terrafor...

A lot of commands used `c.Meta.flagSet()` to create the initial flagset for the command, while quite a few of them didn’t actually use or support the flags that are then added.

So I updated a few commands to use `flag.NewFlagSet()` instead to only add the flags that are actually needed/supported.

Additionally this prevents a few commands from using locking while they actually don’t need locking (as locking is enabled as a default in `c.Meta.flagSet()`.

This work was done against APIs that were already changed in the branch
before work began, and so it doesn't apply to the v0.12 development work.

To allow v0.12 to merge down to master, we'll revert this work out for now
and then re-introduce equivalent functionality in later commits that works
against the new APIs.

This reinstates an old behavior that was lost in the reorganization of how
we deal with the -var and -var-file options.

This fix is verified by TestApply_planVars now passing.

This connects a missing link left by earlier refactoring: the command
package is responsible for gathering up variable values provided by the
user and passing them through to the backend to use in operations.

If we fail to parse the resource address given to "terraform import" then
it's helpful to produce a "source code" snippet of what the user provided
so they might see more precisely which part of the address was invalid.

The "config" package is no longer used and will be removed as part
of the 0.12 release cleanup. Since configschema is part of the
"new world" of configuration modelling, it makes more sense for
it to live as a subdirectory of the newer "configs" package.

Due to how deeply the configuration types go into Terraform Core, there
isn't a great way to switch out to HCL2 gradually. As a consequence, this
huge commit gets us from the old state to a _compilable_ new state, but
does not yet attempt to fix any tests and has a number of known missing
parts and bugs. We will continue to iterate on this in forthcoming
commits, heading back towards passing tests and making Terraform
fully-functional again.

The three main goals here are:
- Use the configuration models from the "configs" package instead of the
  older models in the "config" package, which is now deprecated and
  preserved only to help us write our migration tool.
- Do expression inspection and evaluation using the functionality of the
  new "lang" package, instead of the Interpolator type and related
  functionality in the main "terraform" package.
- Represent addresses of various objects using types in the addrs package,
  rather than hand-constructed strings. This is not critical to support
  the above, but was a big help during the implementation of these other
  points since it made it much more explicit what kind of address is
  expected in each context.

Since our new packages are built to accommodate some future planned
features that are not yet implemented (e.g. the "for_each" argument on
resources, "count"/"for_each" on modules), and since there's still a fair
amount of functionality still using old-style APIs, there is a moderate
amount of shimming here to connect new assumptions with old, hopefully in
a way that makes it easier to find and eliminate these shims later.

I apologize in advance to the person who inevitably just found this huge
commit while spelunking through the commit history.

This is a rather-messy, complex change to get the "command" package
building again against the new backend API that was updated for
the new configuration loader.

A lot of this is mechanical rewriting to the new API, but
meta_config.go and meta_backend.go in particular saw some major
changes to interface with the new loader APIs and to deal with
the change in order of steps in the backend API.

The new config loader requires some steps to happen in a different
order, particularly in regard to knowing the schema in order to
decode the configuration.

Here we lean directly on the configschema package, rather than
on helper/schema.Backend as before, because it's generally
sufficient for our needs here and this prepares us for the
helper/schema package later moving out into its own repository
to seed a "plugin SDK".

We need to share a single config loader across all callers because that
allows us to maintain the source code cache we'll use for snippets in
error messages.

Nothing calls this yet. Callers will be gradually updated away from Module
and Config in subsequent commits.