Because "moved" blocks produce changes that span across more than one
resource instance address at the same time, we need to take extra care
with them during planning.
The -target option allows for restricting Terraform's attention only to
a subset of resources when planning, as an escape hatch to recover from
bugs and mistakes.
However, we need to avoid any situation where only one "side" of a move
would be considered in a particular plan, because that'd create a new
situation that would be otherwise unreachable and would be difficult to
recover from.
As a compromise then, we'll reject an attempt to create a targeted plan if
the plan involves resolving a pending move and if the source address of
that move is not included in the targets.
Our error message offers the user two possible resolutions: to create an
untargeted plan, thus allowing everything to resolve, or to add additional
-target options to include just the existing resource instances that have
pending moves to resolve.
This compromise recognizes that it is possible -- though hopefully rare --
that a user could potentially both be recovering from a bug or mistake at
the same time as processing a move, if e.g. the bug was fixed by upgrading
a module and the new version includes a new "moved" block. In that edge
case, it might be necessary to just add the one additional address to
the targets rather than removing the targets altogether, if creating a
normal untargeted plan is impossible due to whatever bug they're trying to
recover from.
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