Add support for moving symbols across packages with `dart fix`

[natebosch]

Possibly a duplicate of #44892, or may also be considered an extension.

We'd like to move some members across packages, and stop exporting others. The user visible changes is that where they previously had a single import to package:test/test.dart they will now also need a dependency on package:matcher/expect.dart as well as an extra pubspec dependency.

If we can't target the diagnostic about the undefined name for the fix we would be OK with a migration where we mark the exports as deprecated for a while so that the fixes can be applied. We'd need support for deprecating exports in that case. #23067

[bwilkerson]

@pq @keertip

[devoncarew]

Do we know who the owner for this item is? We'd like to ship support for this - for tooling to help us roll out the new package:checks - for the next stable SDK release.

[bwilkerson]

@pq and @keertip are actively working on adding this support. We now support the case where the old export of expect has been removed, but not the case where the export has been marked as deprecated. The reason for that is the issue that was recently linked to this issue by @pq.

[devoncarew]

Ah, great to hear! I didn't know this was in progress.

[pq]

Yeah. #23067 is I think the blocker for @natebosch's use case.