Previously the state migration process was using the fallback strict
error check when migrating to or from a Terraform Cloud workspace. This
resulted in an error when running init if the local and remote Terraform
versions did not exactly match.

This was excessively strict. When migrating from a remote Terraform
Cloud workspace to local state, there is no need for a version check at
all, as we cannot break the Terraform Cloud workspace. When migrating
to Terraform Cloud, we should use the more forgiving check, rather than
the strict equality.

This commit fixes both of these cases accordingly, and allows migrating
state to and from Terraform Cloud remote workspaces without errors.

The workspace name can be overridden by setting a TF_WORKSPACE
environment variable. If this is done, we should still validate the
resulting workspace name; otherwise, we could end up with an invalid and
unselectable workspace.

This change updates the Meta.Workspace function to return an error, and
handles that error wherever necessary.

Most of the state package has been deprecated by the states package.
This PR replaces all the references to the old state package that
can be done simply - the low-hanging fruit.

* states: move state.Locker to statemgr

The state.Locker interface was a wrapper around a statemgr.Full, so
moving this was relatively straightforward.

* command: remove unnecessary use of state package for writing local terraform state files

* move state.LocalState into terraform package

state.LocalState is responsible for managing terraform.States, so it
made sense (to me) to move it into the terraform package.

* slight change of heart: move state.LocalState into clistate instead of
terraform

The backend gets to "prepare" the configuration before Configure is
called, in order to validate the values and insert defaults. We don't
want to store this value in the "config state", because it will often
not match the raw config after it is prepared, forcing unecessary
backend migrations during init.

Since PrepareConfig is always called before Configure, we can store the
config value directly, and assume that it will be prepared in the same
manner each time.

This just finishes off the logging added in earlier commits to get all
the way through to the actual migration call.

This test was initially failing because its fixture had a state which our
new state models consider to be "empty", and thus it was not migrated.

After fixing that (by adding an output to the fixture), this revealed a
bug that the lineage was not being persisted through the migration. This
is fixed by using the statemgr.Migrate method instead of writing via the
normal Writer interface, which allows two cooperating state managers to
properly transfer the lineage and serial along with the state snapshot.

This test was using old-style state files as its input, differing only by
lineage. Since lineages are now managed within the state manager itself,
the test can't use that to distinguish the two files and so we put a
different output in each one instead.

This also introduces some TRACE logging to the migration codepaths.
There's some hard-to-follow control flow here and so this extra logging
helps to understand the reason for a particular outcome, and since this
codepath is visited only in "terraform init" anyway it doesn't hurt to
be a bit more verbose here.

If the backend does not support workspaces, we don't have to try to
select a workspace and we should not return an error.

This is a refactored version of the `remote` backend that was initially added to Terraform v0.11.8 which should now be compatible with v0.12.0.

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.

Due to how often the state and plan types are referenced throughout
Terraform, there isn't a great way to switch them out gradually. As a
consequence, this huge commit gets us from the old world to a _compilable_
new world, but still has a large number of known test failures due to
key functionality being stubbed out.

The stubs here are for anything that interacts with providers, since we
now need to do the follow-up work to similarly replace the old
terraform.ResourceProvider interface with its replacement in the new
"providers" package. That work, along with work to fix the remaining
failing tests, will follow in subsequent commits.

The aim here was to replace all references to terraform.State and its
downstream types with states.State, terraform.Plan with plans.Plan,
state.State with statemgr.State, and switch to the new implementations of
the state and plan file formats. However, due to the number of times those
types are used, this also ended up affecting numerous other parts of core
such as terraform.Hook, the backend.Backend interface, and most of the CLI
commands.

Just as with 5861dbf3 before, I apologize
in advance to the person who inevitably just found this huge commit while
spelunking through the commit history.

Certain backends (currently only the `remote` backend) do not support using both the default and named workspaces at the same time.

To make the migration easier for users that currently use both types of workspaces, this commit adds logic to ask the user for a new workspace name during the migration process.

Use the new StateLocker field to provide a wrapper for locking the state
during terraform.Context creation in the commands that directly
manipulate the state.

Simplify the use of clistate.Lock by creating a clistate.Locker
instance, which stores the context of locking a state, to allow unlock
to be called without knowledge of how the state was locked.

This alows the backend code to bring the needed UI methods to the point
where the state is locked, and still unlock the state from an outer
scope.

Provide a NoopLocker as well, so that callers can always call Unlock
without verifying the status of the lock.

Add the StateLocker field to the backend.Operation, so that the state
lock can be carried between the different function scopes of the backend
code. This will allow the backend context to lock the state before it's
read, while allowing the different operations to unlock the state when
they complete.

This reverts commit e88bd74b.

This reverts commit 400f6ca4.

Use the new StateLocker field to provide a wrapper for locking the state
during terraform.Context creation in the commands that directly
manipulate the state.

Simplify the use of clistate.Lock by creating a clistate.Locker
instance, which stores the context of locking a state, to allow unlock
to be called without knowledge of how the state was locked.

This alows the backend code to bring the needed UI methods to the point
where the state is locked, and still unlock the state from an outer
scope.

The existing prompts were worded as if backend configurations were
named, but they can only really be referenced by their type. Change the
wording to reference them as type "X backend". When migrating state,
refer to the backends as the "previously configured" and "newly
configured", since they will often have the same type.

Only check for input twice in the meta.confirm method. This prevents an
errant newline from aborting the run while allowing Terraform to exit if
there is no input available. We don't just check for a tty, since we
still rely on being able to pipe input in for testing.

Remove the redundant confirmation loops in the migration code, and only
use the confirm method.

We're shifting terminology from "environment" to "workspace". This takes
care of some of the main internal API surface that was using the old
terminology, though is not intended to be entirely comprehensive and is
mainly just to minimize the amount of confusion for maintainers as we
continue moving towards eliminating the old terminology.

Feedback after 0.9 was that the term "environment" was confusing due to
it colliding with several other concepts, such as OS environment
variables, a non-aligned Terraform Enterprise concept, and differing ideas
of "environment" within various organizations.

This new term "workspace" is intended to ease some of that confusion. This
term is not used anywhere else in Terraform today, and we expect it to not
be used in a manner that would be confusing within user organizations.

This begins a deprecation cycle for the "terraform env" family of commands,
instead moving to an equivalent set of "terraform workspace" commands.

There are some remaining references to the old "environment" concept in
the code, which will be cleaned up in a separate change. This change is
instead focused on text visible in the UI and wording within code comments
for the benefit of human maintainers of the code.

"destionation" -> "destination"

A couple commits got rebased together here, and it's easier to enumerate
them in a single commit.

Skip copying of states during migration if they are the same state. This
can happen when trying to reconfigure a backend's options, or if the
state was manually transferred. This can fail unexpectedly with locking
enabled.

Honor the `-input` flag for all confirmations (the new test hit some
more). Also unify where we reference the Meta.forceInitCopy and transfer
the value to the existing backendMigrateOpts.force field.

Add fields required to create an appropriate context for all calls to
clistate.Lock.

Add missing checks for Meta.stateLock, where we would attempt to lock,
even if locking should be skipped.

- Have the ui Lock helper use state.LockWithContext.
- Rename the message package to clistate, since that's how it's imported
  everywhere.
- Use a more idiomatic placement of the Context in the LockWithContext
  args.

The `-force-copy` option will suppress confirmation for copying state
data.

Modify some tests to use the option, making sure to leave coverage of
the Input code path.

Fixes #12871

We were forgetting to remove the legacy remote state from the actual
state value when migrating. This only causes an issue when saving a plan
since the plan contains the state itself and causes an error where both
a backend + legacy state exist.

If saved plans aren't used this causes no noticable issue.

Due to buggy upgrades already existing in the wild, I also added code to
clear the remote section if it exists in a standard unchanged backend

When migrating from a multi-state backend to a single-state backend, we
have to ensure that our locally configured environment is changed back
to "default", or we won't be able to access the new backend.