Historically, we've used TFC's default run messages as a sort of dumping
ground for metadata about the run. We've recently decided to mostly stop
doing that, in favor of:

- Only specifying the run's source in the default message.
- Letting TFC itself handle the default messages.

Today, the remote backend explicitly sets a run message, overriding
any default that TFC might set. This commit removes that explicit message
so we can allow TFC to sort it out.

This shouldn't have any bad effect on TFE out in the wild, because it's
known how to set a default message for remote backend runs since late 2018.

Seems like we lost a newline in some of the shuffling it took to get this
into the live website, and so it's formatting oddly in the rendered
website. This restores the intended formatting of this as the start of
a bullet list, rather than as a continuation of the previous paragraph.

Merge pull request #28864 from hashicorp/alisdair/fix-remote-backend-multi-workspace-state-migration

Fix remote backend multi workspace state migration

* clarify input variables opening sentence

* adjust variables description

* claraify providers text and add learn callout

* add description to providers page

* add desscription and clarify provider configuration

* add deprecation note to versions in proivder configs

* add hands on callout and clarify next steps in intro

* link to language collection from language docs

* give more context about configurtion language up front

* clarify output top page

* reorganize for each intro to present feature before notes

* move description before link out and remove passive voice

* fix typo

* clarify purpose of plan

* move explanation before learn link and fully spell boolean

* add a syntax heading  to separate intro from details

* add learn callout to module source docs

* clean up intro to provider requirements and add link

* Apply suggestions from code review
Co-authored-by: Tu Nguyen <im2nguyen@users.noreply.github.com>

* Apply suggestions from code review
Co-authored-by: Tu Nguyen <im2nguyen@users.noreply.github.com>
Co-authored-by: Tu Nguyen <im2nguyen@users.noreply.github.com>

* tools: remove terraform-bundle.

terraform-bundle is no longer supported in the main branch of terraform. Users can build terraform-bundle from terraform tagged v0.15 and older.

* add a README pointing users to the v0.15 branch

Previously we had a separation between ModuleSourceRemote and
ModulePackage as a way to represent within the type system that there's an
important difference between a module source address and a package address,
because module packages often contain multiple modules and so a
ModuleSourceRemote combines a ModulePackage with a subdirectory to
represent one specific module.

This commit applies that same strategy to ModuleSourceRegistry, creating
a new type ModuleRegistryPackage to represent the different sort of
package that we use for registry modules. Again, the main goal here is
to try to reflect the conceptual modelling more directly in the type
system so that we can more easily verify that uses of these different
address types are correct.

To make use of that, I've also lightly reworked initwd's module installer
to use addrs.ModuleRegistryPackage directly, instead of a string
representation thereof. This was in response to some earlier commits where
I found myself accidentally mixing up package addresses and source
addresses in the installRegistryModule method; with this new organization
those bugs would've been caught at compile time, rather than only at
unit and integration testing time.

While in the area anyway, I also took this opportunity to fix some
historical confusing names of fields in initwd.ModuleInstaller, to be
clearer that they are only for registry packages and not for all module
source address types.

We have some tests in this package that install real modules from the real
registry at registry.terraform.io. Those tests were written at an earlier
time when the registry's behavior was to return the URL of a .tar.gz
archive generated automatically by GitHub, which included an extra level
of subdirectory that would then be reflected in the paths to the local
copies of these modules.

GitHub started rate limiting those tar archives in a way that Terraform's
module installer couldn't authenticate to, and so the registry switched
to returning direct git repository URLs instead, which don't have that
extra subdirectory and so the local paths on disk now end up being a
little different, because the actual module directories are at a different
subdirectory of the package.

Now that we (in the previous commit) refactored how we deal with module
sources to do the parsing at config loading time rather than at module
installation time, we can expose a method to centralize the determination
for whether a particular module call (and its resulting Config object)
enters a new external package.

We don't use this for anything yet, but in later commits we will use this
for some cross-module features that are available only for modules
belonging to the same package, because we assume that modules grouped
together in a package can change together and thus it's okay to permit a
little more coupling of internal details in that case, which would not
be appropriate between modules that are versioned separately.

It's been a long while since we gave close attention to the codepaths for
module source address parsing and external module package installation.
Due to their age, these codepaths often diverged from our modern practices
such as representing address types in the addrs package, and encapsulating
package installation details only in a particular location.

In particular, this refactor makes source address parsing a separate step
from module installation, which therefore makes the result of that parsing
available to other Terraform subsystems which work with the configuration
representation objects.

This also presented the opportunity to better encapsulate our use of
go-getter into a new package "getmodules" (echoing "getproviders"), which
is intended to be the only part of Terraform that directly interacts with
go-getter.

This is largely just a refactor of the existing functionality into a new
code organization, but there is one notable change in behavior here: the
source address parsing now happens during configuration loading rather
than module installation, which may cause errors about invalid addresses
to be returned in different situations than before. That counts as
backward compatible because we only promise to remain compatible with
configurations that are _valid_, which means that they can be initialized,
planned, and applied without any errors. This doesn't introduce any new
error cases, and instead just makes a pre-existing error case be detected
earlier.

Our module registry client is still using its own special module address
type from registry/regsrc for now, with a small shim from the new
addrs.ModuleSourceRegistry type. Hopefully in a later commit we'll also
rework the registry client to work with the new address type, but this
commit is already big enough as it is.

We've previously had the syntax and representation of module source
addresses pretty sprawled around the codebase and intermingled with other
systems such as the module installer.

I've created a factored-out implementation here with the intention of
enabling some later refactoring to centralize the address parsing as part
of configuration decoding, and thus in turn allow the parsing result to
be seen by other parts of Terraform that interact with configuration
objects, so that they can more robustly handle differences between local
and remote modules, and can present module addresses consistently in the
UI.

This new package aims to encapsulate all of our interactions with
go-getter to fetch remote module packages, to ensure that the rest of
Terraform will only use the small subset of go-getter functionality that
our modern module installer uses.

In older versions of Terraform, go-getter was the entire implementation
of module installation, but along the way we found that several aspects of
its design are poor fit for Terraform's needs, and so now we're using it
as just an implementation detail of Terraform's handling of remote module
packages only, hiding it behind this wrapper API which exposes only
the services that our module installer needs.

This new package isn't actually used yet, but in a later commit we will
change all of the other callers to go-getter to only work indirectly
through this package, so that this will be the only package that actually
imports the go-getter packages.

As the comment notes, this hostname is the default for provide source
addresses. We'll shortly be adding some address types to represent module
source addresses too, and so we'll also have DefaultModuleRegistryHost
for that situation.

(They'll actually both contain the the same hostname, but that's a
coincidence rather than a requirement.)

Previously this is a task that I've just taken on using my own knowledge
of how these parts fit together, but that's not at all a sustainable
situation, and so here I've just documented the steps I've followed in for
previous Unicode version upgrades, and thus which I'd expect to follow
for future Unicode version upgrades too.

I hope this is also a somewhat-interesting overview of how Terraform makes
use of Unicode, even for folks who are just working on the other relevant
features of Terraform and not specifically trying to change Terraform's
Unicode support.

Since I'm the primary maintainer of two of the dependencies involved in
this it seems likely that following this process will still involve me
in _some_ capacity, but hopefully only necessarily in the form of
reviewing PRs and cutting new releases of those libraries.

Fix support link (mailto is not supported in GitHub template URLs)

Add support reference for TFC-related issues

When performing state migration to a remote backend target, Terraform
may fail due to mismatched remote and local Terraform versions. Here we
add the `-ignore-remote-version` flag to allow users to ignore this
version check when necessary.

When migrating multiple local workspaces to a remote backend target
using the `prefix` argument, we need to perform the version check
against all existing workspaces returned by the `Workspaces` method.
Failing to do so will result in a version check error.

Revert "mclarify specifying provider versions"

This reverts commit 397494da.

Our module installer has a somewhat-informal idea of a "module package",
which is some external thing we can go fetch in order to add one or more
modules to the current configuration. Our documentation doesn't talk much
about it because most users seem to have found the distinction between
external and local modules pretty intuitive without us throwing a lot of
funny terminology at them, but there are some situations where the
distinction between a module and a module package are material to the
end-user.

One such situation is when using an absolute rather than relative
filesystem path: we treat that as an external package in order to make the
resulting working directory theoretically "portable" (although users can
do various other things to defeat that), and so Terraform will copy the
directory into .terraform/modules in the same way as it would download and
extract a remote archive package or clone a git repository.

A consequence of this, though, is that any relative paths called from
inside a module loaded from an absolute path will fail if they try to
traverse upward into the parent directory, because at runtime we're
actually running from a copy of the directory that's been taking out of
its original context.

A similar sort of situation can occur in a truly remote module package if
the author accidentally writes a "../" source path that traverses up out
of the package root, and so this commit introduces a special error message
for both situations that tries to be a bit clearer about there being a
package boundary and use that to explain why installation failed.

We would ideally have made escaping local references like that illegal in
the first place, but sadly we did not and so when we rebuilt the module
installer for Terraform v0.12 we ended up keeping the previous behavior of
just trying it and letting it succeed if there happened to somehow be a
matching directory at the given path, in order to remain compatible with
situations that had worked by coincidence rather than intention. For that
same reason, I've implemented this as a replacement error message we will
return only if local module installation was going to fail anyway, and
thus it only modifies the error message for some existing error situations
rather than introducing new error situations.

This also includes some light updates to the documentation to say a little
more about how Terraform treats absolute paths, though aiming not to get
too much into the weeds about module packages since it's something that
most users can get away with never knowing.

backend/remote: point refresh users towards -refresh-only

backend/local: Fix lock when applying stale plan

When returning from the context method, a deferred function call checked
for error diagnostics in the `diags` variable, and unlocked the state if
any exist. This means that we need to be extra careful to mutate that
variable when returning errors, rather than returning a different set of
diags in the same position.

Previously this would result in an invalid plan file causing a lock to
become stuck.

We got the replacement for this in earlier than anticipated, so these docs
were originally more pessimistic about when the alternative would be
available.

While we were working on and documenting these it wasn't clear exactly
what Terraform CLI version they would land in, and so we used
"Terraform v1.0" in the docs as a safe bound that was definitely going to
include all of them.

With everything now landed though, we can be more specific about which
v0.15.x minor release each of these appeared in.

website: Add documentation for machine readable UI

command/views: Remove unused drift summary message

return error for invalid resource import

Most legacy provider resources do not implement any import functionality
other than returning an empty object with the given ID, relying on core
to later read that resource and obtain the complete state. Because of
this, we need to check the response from ReadResource for a null value,
and use that as an indication the import id was invalid.