Our previous rule for implicitly moving from IntKey(0) to NoKey would
apply that move even when the current resource configuration uses
for_each, because we were only considering whether "count" were set.

Previously this was relatively harmless because the resource instance in
question would end up planned for deletion anyway: neither an IntKey nor
a NoKey are valid keys for for_each.

Now that we're going to be announcing these moves explicitly in the UI,
it would be confusing to see Terraform report that IntKey moved to NoKey
in a situation where the config changed from count to for_each, so to
address that we'll only generate the implied statement if neither
repetition argument is set.

There are a few different reasons why a resource instance tracked in the
prior state might be considered an "orphan", but previously we reported
them all identically in the planned changes.

In order to help users understand the reason for a surprising planned
delete, we'll now try to specify an additional reason for the planned
deletion, covering all of the main reasons why that could happen.

This commit only introduces the new detail to the plans.Changes result,
though it also incidentally exposes it as part of the JSON plan result
in order to keep that working without returning errors in these new
cases. We'll expose this information in the human-oriented UI output in
a subsequent commit.

The core runtime is now able to specify a reason for some situations when
Terraform plans to delete a resource instance.

This commit makes that information visible in the human-oriented UI. A
previous commit already made the underlying data informing these new hints
visible as part of the machine-oriented (JSON) plan output.

This also removes the bold formatting from the existing "has moved to"
hints, because subjectively it seemed like the result was emphasizing too
many parts of the output and thus somewhat defeating the benefit of the
emphasis in trying to create additional visual hierarchy for sighted users
running Terraform in a terminal. Now only the first line containing the
main action statement will be in bold, and all of the parenthesized
follow-up notes will be unformatted.

In most cases Terraform will be able to automatically fully resolve all
of the pending move statements before creating a plan, but there are some
edge cases where we can end up wanting to move one object to a location
where another object is already declared.

One relatively-obvious example is if someone uses "terraform state mv" in
order to create a set of resource instance bindings that could never have
arising in normal Terraform use.

A less obvious example arises from the interactions between moves at
different levels of granularity. If we are both moving a module to a new
address and moving a resource into an instance of the new module at the
same time, the old module might well have already had a resource of the
same name and so the resource move will be unresolvable.

In these situations Terraform will move the objects as far as possible,
but because it's never valid for a move "from" address to still be
declared in the configuration Terraform will inevitably always plan to
destroy the objects that didn't find a final home. To give some additional
explanation for that result, here we'll add a warning which describes
what happened.

This is not a particularly actionable warning because we don't really
have enough information to guess what the user intended, but we do at
least prompt that they might be able to use the "terraform state" family
of subcommands to repair the ambiguous situation before planning, if they
want a different result than what Terraform proposed.

Because our validation rules depend on some dynamic results produced by
actually running the plan, we deal with moves in a "backwards" order where
we try to apply them first -- ignoring anything strange we might find --
and then validate the original statements only after planning.

An unfortunate consequence of that approach is that when the move
statements are invalid it's likely that move execution will not fully
complete, and so the generated plan is likely to be incorrect and might
well include errors resulting from the unresolved moves.

To mitigate that, here we let any move validation errors supersede all
other diagnostics that the plan phase might've generated, in the hope that
it'll help the user focus on fixing the incorrect move statements without
creating confusing by reporting errors that only appeared as a quick of
how Terraform worked around the invalid move statements earlier.