Fix runtime vendoring bytecode compilation races.

[jsirois]

Fixes #1098

[jsirois]

The remaining os.walks in Pex are in:

1. pex/vendor/__main__.py: Not a production component.
2. pex/pex_builder.py: The use here is safe since the walk is always over a distribution (zipped or installed wheel), and the action is a copy into a different location - the PEXBuilder chroot. As such executions (and thus buytecode compilations) occur in a different place from the source distribution file(s).
3. pex/bin/pex.py: Only used to gather --sources-directory and --resources-directory. The distinction there has never been relevant and should be removed and, as such, we cannot filter out pyc which could be valid resources.