Replace exclusives_groups with a compile_classpath product

As discussed [on the mailing list](https://groups.google.com/d/topic/pants-devel/3cBrJ_rZfCI/discussion).

- Replace all usage of exclusives and ExclusivesMapping with an OrderedSet containing the compile_classpath
- Don't reverse resources targets while appending them to the classpath in prepare_resources.py

Testing Done:
PANTS_DEV=1 ./pants test tests/python::

Reviewed at https://rbcommons.com/s/twitter/r/1539/