Upgrade cty to v1.8.4

This PR adds decoding for the upcoming "moved" blocks in configuration. This code is gated behind an experiment called EverythingIsAPlan, but the experiment is not registered as an active experiment, so it will never run (there is a test in place which will fail if the experiment is ever registered).

This also adds a new function to the Targetable interface, AddrType, to simplifying comparing two addrs.Targetable.

There is some validation missing still: this does not (yet) descend into resources to see if the actual resource types are the same (I've put this off in part because we will eventually need the provider schema to verify aliased resources, so I suspect this validation will have to happen later on).

* docs: Document naming conventions for templates & backend configs

* Update website/docs/cli/config/config-file.html.md
Co-authored-by: Alisdair McDiarmid <alisdair@users.noreply.github.com>

* Update website/docs/language/functions/templatefile.html.md
Co-authored-by: Alisdair McDiarmid <alisdair@users.noreply.github.com>
Co-authored-by: Alisdair McDiarmid <alisdair@users.noreply.github.com>

json-output: Omit unchanged resource_drift entries

blocktoattr fixup dropping MissingItemRange

When blocktoattr.fixupBody returned its content, the value for
`MissingItemRange` was omitted, losing the diagnostic Subject.

Previously, if any resources were found to have drifted, the JSON plan
output would include a drift entry for every resource in state. This
commit aligns the JSON plan output with the CLI UI, and only includes
those resources where the old value does not equal the new value---i.e.
drift has been detected.

Also fixes a bug where the "address" field was missing from the drift
output, and adds some test coverage.

* command: new command, terraform add, generates resource templates

terraform add ADDRESS generates a resource configuration template with all required (and optionally optional) attributes set to null. This can optionally also pre-populate nonsesitive attributes with values from an existing resource of the same type in state (sensitive vals will be populated with null and a comment indicating sensitivity)

* website: terraform add documentation

* Quoting filesystem path in scp command argument

* Adding proper shell quoting for scp commands

* Running go fmt

* Using a library for quoting shell commands

* Don't export quoteShell function

handle unexpected changes to unknown block

The "references" included in the expression representation now properly unwrap for each traversal step, to match what was documented.

* jsonplan and jsonstate: include sensitive_values in state representations

A sensitive_values field has been added to the resource in state and planned values which is a map of all sensitive attributes with the values set to true.

It wasn't entirely clear to me if the values in state would suffice, or if we also need to consult the schema - I believe that this is sufficient for state files written since v0.15, and if that's incorrect or insufficient, I'll add in the provider schema check as well.

I also updated the documentation, and, since we've considered this before, bumped the FormatVersions for both jsonstate and jsonplan.

* website/docs: move type func docs to a useful location

* docs don't exist if you don't put them in the index (again)

An unknown block represents a dynamic configuration block with an
unknown for_each value. We were not catching the case where a provider
modified this value unexpectedly, which would crash with block of type
NestingList blocks where the config value has no length for comparison.

account for noop deposed instances in json plan

When rendering a json plan, we need to account for deposed instances
that have become a noop rather than a destroy.

Fixed a typo.

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...