Add a flexible directory re-mapper for the bundle.

Testing Done:
PANTS_DEV=1 ./pants goal test tests/python/pants_test:all
./build-support/bin/ci.sh

Bugs closed: 701

Reviewed at https://rbcommons.com/s/twitter/r/1181/
Sync of https://github.com/pantsbuild/pants/commit/7b39eac93d839acd0cc3f9123688a01e3a20ae3d