Make error_prone_annotations dependency optional

[HannesWell]

At the moment the error_prone_annotations are a mandatory requirement. But annotations with retention policy 'runtime' are ignored if they cannot be loaded at runtime (at least according to Stack-Overvflow). This PR aims to make the dependency optional to not enforce the presence of error_prone_annotations in a java runtime if those annotations are not used at all.

This is especially useful for OSGi-runtimes where you cannot exclude error_prone_annotations because the corresponding package is specified as mandatory requirement in the MANIFEST.MF. Marking the Maven dependency as optional will also result in the error-prone package requirement being marked as optional.

At the moment error_prone_annotations does not come with a OSGi compliant Manifest, which makes it harder to include it in OSGi applications like Eclipse.
I already created a PR at error-prone to include the required OSGi headers (see google/error-prone#3903), but not requiring the annotations at all would make it even simpler to use guice within OSGi.

Alternatively the dependency could only be marked as optional in the OSGi metadata.

[sameb]

Thanks @HannesWell . If an alternative is proper OSGi info in error prone, could you update a PR there for doing that and let me know when it's updated? I can try to nudge folks internally.

[HannesWell]

Thanks @HannesWell . If an alternative is proper OSGi info in error prone, could you update a PR there for doing that and let me know when it's updated? I can try to nudge folks internally.

The PR is already there, it is google/error-prone#3903
It would be great if you make them aware of it. Thanks in advance. :)

[sameb]

(FYI, I locally confirmed that running binaries that reference the errorprone annotations, even reflectively iterating through annotations & annotation of annotations) works just fine if the annotation isn't on the classpath at runtime.)

[HannesWell]

(FYI, I locally confirmed that running binaries that reference the errorprone annotations, even reflectively iterating through annotations & annotation of annotations) works just fine if the annotation isn't on the classpath at runtime.)

Great, thanks for that.
And thanks a lot for reviewing and merging this PR just in time for Guice 6/7 as well as for the help in error-prone (I assume you reached your college).