Add support for moving classes between libraries

[natebosch]

This is a follow on to #48997

We can define a fix which uses replacedBy to move functions and variables to a new import, but cannot do the same with classes.

Can we either support replacedBy with classes, or add a movedTo operation which maintains the name and replaces only the import URI.

See dart-lang/test#1978 (comment)

[scheglov]

@bwilkerson @keertip

[natebosch]

This is coming up again in the context of a package rename for firebase_vertexai. It would be very helpful if we can re-export the code from the old package with a deprecation that helps folks migrate to the new one. We can't use the fix without support for moving the classes, so for now my plan will be to add dart fix data that helps with the renames and then inform users to manually rewrite pubspecs and imports.