Add targets for building individual modules using javac

This is needed in order to be able to run error prone (which is
a plugin replacement for javac) on a subset of the java code.

Bug: 31507496
Test: Build javac-check-core-tests and check the resulting JAR
Change-Id: I5a556920572fcdad7b39d67e3f00fe0e0f1f8f46