Add a TargetKind for annotations on imports, exports, and part directives.

[natebosch]

This might be blocked on changing TargetKind into a class with static constants instead of an enum as discussed in #49796

The recommended pattern for using @GenerateNiceMocks from package:mockito is to annotate the import of the generated file. This is a pattern we are likely to repeat into other packages.

It would be great if we can statically require the annotation on an import or export. I think it could be fine to have a single TargetKind.directive, but it may be more useful to split it up into imports, exports, and parts.

[matanlurey]

If you did a bit mask you could do something cute like:

static const anyDirective = TargetKind.importDirective | TargetKind.exportDirective | ...