refactoring: allow cross-package move statements

[kmoe]

Remove validation errors on cross-package move statements.

[apparentlymart]

This looks good to me!

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I wonder if we should replace the now-obsolete paragraph in the documentation with some guidance about how to safely implement a cross-package move. 🤔

For example, I think we'd planned to recommend setting at least a minimum version pin on the module call to make sure the child module is at least new enough to either contain the resource(s) being moved to or to not contain the resource(s) being moved from. It follows from that advice that one must publish the new version of the package containing the child module that has the expected additions/removals before adding the upstream moved blocks.

I think there were some other items of guidance we discussed during design work too, but I'm not recalling them off the top of my head right now.

I'm admittedly not really sure where best to introduce this information and how to structure it. I don't think just inserting it directly in place of the removed paragraph would make sense because we'll probably have more than just the one short paragraph to include, but perhaps we could add a new section later in the page and link to it as an additional sentence on the end of the paragraph just before, which mentions that this example is for moves between modules in a single package.

No harm in looking into this in a separate PR though, since this PR won't leave the documentation incorrect, and the guidance we'd add does follow logically from other Terraform behaviors and thus experienced Terraform users can probably figure it out based on what they already know.

[github-actions]

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[github-actions]

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