getdeps: make several improvements to the eden manifest
getdeps: make several improvements to the eden manifest

Summary:
- Indicate that the EDEN_VERSION_OVERRIDE environment variable affects the
  build.
- Exclude eden/config.py from the shipit  path map, as this file is
  auto-generated in open source builds.
- Include pexpect as a dependency, as this is needed for some of the
  integration tests.

Reviewed By: genevievehelsel

Differential Revision: D21000163

fbshipit-source-id: 8eec378f66487229c995f637c4787eae530c7845

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift