Fix pex force local to handle PEP 420.

[jsirois]

Previously, pex would blow up trying to adjust import paths if a PEP-420
implicit namespace package was encountered. Add a test of the path
adjustment functionality both to confirm it was needed (it was) and that
the fix technique works with all forms of namespace packages by only
assuming they support append.

Fixes #598

[jsirois]

NB: This is feathered off of #612 which was required to provide stable behavior of setuptools (pkg_resources) namespace packages which varied in at least 3 ways over versions of setuptools PEX previously claimed to support.

Reviewable changes are contained in beb77bd

[jsirois]

Rebased against master now that vendoring change is in. This is just a CI burn, one more cleanup pass and I'll ping reviewers for another look.

[jsirois]

Alright - ready for re-review. Thanks again in advance!

[stuhood]

Thanks!

[Eric-Arellano]

This is great to see PEP 420 support! Thanks for adding this.

[jsirois]

Noting the OSX errors look like the classic, befuddling /private/var/folders vs /var/folders - os.path.realpath needed - thing:

File "/var/folders/nz/vv4_9tw56nv9k3tkvyszvwg80000gn/T/tmp8ecs_u41/pex2/.bootstrap/pex/compatibility.py", line 81, in exec_function
    exec(ast, globals_map, locals_map)
  File "/private/var/folders/nz/vv4_9tw56nv9k3tkvyszvwg80000gn/T/tmp8ecs_u41/pex2/.deps/foo_bar_baz-0.0.1-py3-none-any.whl/bin/foobaz", line 6, in <module>
    sys.exit(baz.main())

[Eric-Arellano]

Changes look great. This is a very high quality PR. Thanks John! 🎉

[jsirois]

Thanks for the reviews and suggestions folks! Green and merging.