This commit makes a couple changes to allow staleness_test() to be used
from outside the upb repo:
- Fully qualify references to upb targets and wrap them in a Label()
constructor. See here for details:
https://bazel.build/extending/macros#label-resolution
- Make the :staleness_test_lib target public.
This renaming is something we have been planning on doing, and I would
like to do it now because I'm getting ready to rely on this
staleness_test() macro from the main protobuf repo.
From now on, these files will live in the "generated" branch only, and a GitHub action will regenerate these files whenever there is a commit to the main branch.
PiperOrigin-RevId: 438879338