Rebase onto eisop

[cpovirk]

That should pick up at least a couple improvements:

• better support for T extends @Nullable Object
• an easier way to support package-level annotations that don't affect subpackages

I may be forgetting others.

[cpovirk]

(Arguably this issue belongs in https://github.com/jspecify/checker-framework/issues, not here.)

[edit: Done! See below. (It used to be jspecify/jspecify-reference-checker#30) Thanks to netdpb for finding the "Transfer issue" button.]