As the Graph is walked, the current way to set the context path was to
have the walker return a context from EnterPath. This required that
every node know it's absolute path, which can no longer be the case
during plan when modules have not been expanded.

This introduces a new method called WithPath, which returns a copy of
the context with the internal path updated to reflect the method
argument. Any use of the EvalContext that requires knowing the path will
now panic if it wasn't explicitly set to ensure that evaluations always
occur in the correct path.

Add EvalContext to the GraphWalker interface.
EvalContext returns an EvalContext that has not yet set a path. This
will allow us to enforce that all context operations requiring a module
instance path will require that a path be explicitly set rather than
evaluating within the wrong path.

This was incorrectly removing the _source_ entry prior to creating the
symlink, therefore ending up with a dangling symlink and no source file.

This wasn't obvious before because the test case for LinkFromOtherCache
was also incorrectly named and therefore wasn't running. Fixing the name
of that test made this problem apparent.

The TestLinkFromOtherCache test case now ends up seeing the final resolved
directory rather than the symlink target, because of upstream changes
to the internal/getproviders filesystem scanning logic to handle symlinks
properly.

Previously this was failing to treat symlinks to directories as unpacked
layout, because our file info was only an Lstat result, not a full Stat.

Now we'll resolve the symlink first, allowing us to handle a symlink to
a directory. That's important because our internal/providercache behavior
is to symlink from one cache to another where possible.

There's a lot going on in these functions that can be hard to follow from
the outside, so we'll add some additional trace logging so that we can
more easily understand why things are behaving the way they are.

This was accidentally left out of an earlier commit due to our top-level
.gitignore file containing *.exe as an ignore pattern.

Expander.ExpandResource cannot expand all modules

When a provider source produces an HTTP URL location we'll expect it to
resolve to a zip file, which we'll first download to a temporary
directory and then treat it like a local archive.

When a provider source produces a local archive path we'll expect it to
be a zip file and extract it into the target directory.

This does not yet include an implementation of installing from an
already-unpacked local directory. That will follow in a subsequent commit,
likely following a similar principle as in Dir.LinkFromOtherCache.

These new functions allow command implementations to get hold of the
providercache objects and installation source object derived from the
current CLI configuration.

The MultiSource isn't actually properly implemented yet, but this is a
minimal implementation just for the case where there are no underlying
sources at all, because we use an empty MultiSource as a placeholder
when a test in the "command" package fails to explicitly populate a
ProviderSource.

This is not tested yet, but it's a compilable strawman implementation of
the necessary sequence of events to coordinate all of the moving parts
of running a provider installation operation.

This will inevitably see more iteration in later commits as we complete
the surrounding parts and wire it up to be used by "terraform init". So
far, it's just dead code not called by any other package.

The Installer type will encapsulate the logic for running an entire
provider installation request: given a set of providers to install, it
will determine a method to obtain each of them (or detect that they are
already installed) and then take the necessary actions.

So far it doesn't do anything, but this stubs out an interface by which
the caller can request ongoing notifications during an installation
operation.

This will eventually be responsible for actually retrieving a package from
a source and then installing it into the cache directory, but for the
moment it's just a stub to complete the proposed API, which I intend to
test in a subsequent commit by writing the full "Installer" API that will
encapsulate the full installation logic.

When a system-wide shared plugin cache is configured, we'll want to make
use of entries already in the shared cache when populating a local
(configuration-specific) cache.

This new method LinkFromOtherCache encapsulates the work of placing a link
from one cache to another. If possible it will create a symlink, therefore
retaining a key advantage of configuring a shared plugin cache, but
otherwise we'll do a deep copy of the package directory from one cache
to the other.

Our old provider installer would always skip trying to create symlinks on
Windows because Go standard library support for os.Symlink on Windows
was inconsistent in older versions. However, os.Symlink can now create
symlinks using a new API introduced in a Windows 10 update and cleanly
fail if symlink creation is impossible, so it's safe for us to just
try to create the symlink and react if that produces an error, just as we
used to do on non-Windows systems when possibly creating symlinks on
filesystems that cannot support them.

The existing functionality in this package deals with finding packages
that are either available for installation or already installed. In order
to support installation we also need to determine the location where a
package should be installed.

This lives in the getproviders package because that way all of the logic
related to the filesystem layout for local provider directories lives
together here where they can be maintained together more easily in future.

We've previously been copying this function around so it could remain
unexported while being used in various packages. However, it's a
non-trivial function with lots of specific assumptions built into it, so
here we'll put it somewhere that other packages can depend on it _and_
document the assumptions it seems to be making for future reference.

As a bonus, this now uses os.SameFile to detect when two paths point to
the same physical file, instead of the slightly buggy local implementation
we had before which only worked on Unix systems and did not correctly
handle when the paths were on different physical devices.

The copy of the function I extracted here is the one from internal/initwd,
so this commit also includes the removal of that unexported version and
updating the callers in that package to use at at this new location.

Historically our logic to handle discovering and installing providers has
been spread across several different packages. This package is intended
to become the home of all logic related to what is now called "provider
cache directories", which means directories on local disk where Terraform
caches providers in a form that is ready to run.

That includes both logic related to interrogating items already in a cache
(included in this commit) and logic related to inserting new items into
the cache from upstream provider sources (to follow in later commits).

These new codepaths are focused on providers and do not include other
plugin types (provisioners and credentials helpers), because providers are
the only plugin type that is represented by a heirarchical, decentralized
namespace and the only plugin type that has an auto-installation protocol
defined. The existing codepaths will remain to support the handling of
the other plugin type...

Previously this was available by instantiating a throwaway
FilesystemMirrorSource, but that's pretty counter-intuitive for callers
that just want to do a one-off scan without retaining any ongoing state.

Now we expose SearchLocalDirectory as an exported function, and the
FilesystemMirrorSource then uses it as part of its implementation too.
Callers that just want to know what's available in a directory can call
SearchLocalDirectory directly.

ExpandResource must only check the specific ModuleInstances in the
requested path, because the resource may not have been registered yet in
all module instances.

Expander.ExpandResource should use AbsResource

This also calls ExpandModuleResource in one location, because the logic
is not yet updated to handle actual module expansion, but that will be
fixed in a forthcoming PR.

It turns out that within terraform resource expansion will normally
happen with an absolute address. This is because in order to evaluate
the expansion expression, we need to already have the context within a
module instance.

This leaves the existing ExpandResource logic in place as
ExpandModuleResource since it's working, and in case we do find a
location where it's useful to get a full expansion (which may be needed
for instance dependency tracking)

Reword some of the resource related arguments and comments, as they were
copied from the module methods and not entirely accurate.

* add IsLegacy and IsDefault funcs to addrs.Provider
* add some test coverage

missingPlugins was hard-coded to work only with provider plugins, so I
renamed it to clarify the usage.

Also renamed a test provider from greater_than to greater-than as the
underscore is an invalid provider name character and this will become a
hard error in the near future.

* import: remove Config from ImportOpts

`Config` in ImportOpts was any provider configuration provided by the
user on the command line. This option has already been removed in favor
of only taking the provider from the configuration loaded in the current
context.

* terrafrom: add Config to ImportStateTransformer and refactor Transform
to get the resource provider FQN from the Config

registry: Fix panic when server is unreachable

Non-HTTP errors previously resulted in a panic due to dereferencing the
resp pointer while it was nil, as part of rendering the error message.
This commit changes the error message formatting to cope with a nil
response, and extends test coverage.

Fixes #24384

* command: remove 0.12upgrade and related `configupgrade` library
* leave deprecation warning for 0.12upgrade to point users to v0.12

Since these links were in the soon-to-be-deprecated 0.11 language section, I
think we can just remove them without needing to find an equivalent link.

command: Add scaffold for 0.13upgrade command

* terraform: large refactor to use Provider from configs.Resource

configs.Resource.ImpliedProvider() now returns a string; it is the
callers' responsibility to turn that into an addrs.Provider if needed.

GraphNodeProviderConsumer ProvidedBy() no longer returns nil (reverting
to earlier, pre-provider-fqn behavior): it will return either the
provider set in config, provider set in state, or the default provider.

It seems that the checksum for v3.3.10+incompatible has changed at some
point, causing "go mod vendor" to fail now.

We can see by the fact that no files within "vendor" have changed that the
change in checksum is not the result of any material change in the module
code, and therefore presumably resulted from some change in metadata or
a change in the Go module hashing algorithm since Go 1.12.

more module expansion foundation