Relativize classpath for non-ng java execution

-Factored the classpath relativization logic into it's own module and reworked it slightly.
-Made jmake relativize classpath in addition to the nailgun executor

The jvm_utils BUILD file has an empty dependencies key to follow the convention I found in another file.

Testing Done:
Ran a java/scala test target with nailgun enabled/disabled and made sure the classpath was minimized and the tests passed.

Reviewed at https://rbcommons.com/s/twitter/r/804/