@TestOnly should be allowed for ElementType.PACKAGE

I have a maven module which should only be used for tests in other modules.

Therefore, it would be very useful to be able to provide the package with `@TestOnly` in a `package-info.java` and not to provide every single class of the module with it.

Start by reading the @TestOnly declaration and its ElementType targets. Check how package-info.java can apply the annotation, then verify that package-level use is accepted and the existing project checks pass.

Written by the indexing model from the issue text.