Summary:
* Fix a minor issue with `add_fb_python_executable` where it would
  emit `--main` even if `MAIN_MODULE` had not been specified
* Introduce the ability to rename python sources in the manifest so
  that they have a different install path.
* Note that this changes the DEPENDS parameters to reference abs_sources;
  previously these were using relative paths to the sources pulled directly
  from the SOURCES arguments, but since those can now contain aliasing
  operations we need to pre-process the list.  I switched to abs_sources
  because that was already being maintained.  If for some reason the
  DEPENDS list needs to be relative paths, this will need to be adjusted.

Reviewed By: simpkins

Differential Revision: D17147417

fbshipit-source-id: d0825bfbdad6be658c078aaac6645e6926a1530f