[rsc-compile] break out java metacping into a separate job

[baroquebobcat]

Currently java targets are metacp'd as part of building each scala target. It would be better to metacp them once per java target.

There's a couple approaches here

1. create a new Job that

• depends on java zinc compiles
• is depended on by rsc scala compiles
• runs metacp on the result of a java zinc compile

2. wrap the existing java zinc work fn with a fn that runs metacp after invoking the zinc compile. This doesn't require any network changes.

This is very similar to 2be0b1d#diff-074dd871ff62c835ab90d4a0a1264a63

• The remaining metacp portion of work_for_vts_rsc https://github.com/pantsbuild/pants/blob/master/src/python/pants/backend/jvm/tasks/jvm_compile/rsc/rsc_compile.py#L435
• The jar_library metacp fn https://github.com/pantsbuild/pants/blob/master/src/python/pants/backend/jvm/tasks/jvm_compile/rsc/rsc_compile.py#L520

I haven't spent much time refactoring this, so there is a lot of duplication.

[stuhood]

Thank you @baroquebobcat... will tackle this this/next week.

[stuhood]

Argh... sorry. I left this too long, and now looking at it more deeply, I definitely will not be able to complete it in time.

I am definitely, definitely left feeling that we should invest in #6898 or #6856 in order to unblock porting this code to @rules.