plans: indicate when resource deleted due to move

[kmoe]

Add a new ChangeReason, ReasonDeleteBecauseNoMoveTarget, to provide better information in cases where a planned deletion is due to moving a resource to a target not in configuration.

This provides the user an additional clue as to what has happened, in a case in which Terraform has elided a user's action and inaction into one potentially destructive change.

Example

Consider a state which contains the resource null_resource.foo.

Configuration

moved {
  from = null_resource.foo
  to = null_resource.nonexist
}

Steps

1. terraform plan

Before this PR

After this PR

[alisdair]

The UI looks good to me.

I think we could achieve this same UI result without adding a new change reason. Essentially, taking the logic from NodePlannableResourceInstanceOrphan and moving it to the diff renderer. I'm not sure if that's better or worse, but it'd be a smaller diff.

[kmoe]

Thank you. I agree that this could be done in principle in the diff renderer. It would require a bit of refactoring which I think would make the code less clear and harder to maintain, since we would have to add logic to say that in this one particular case, the # (because... text is dependent on something other than the ActionReason.

[github-actions]

Reminder for the merging maintainer: if this is a user-visible change, please update the changelog on the appropriate release branch.

[github-actions]

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