Summary:
Add a new builder that can extract Python wheel files, and re-package them
for consumption by our add_fb_python_library() and add_fb_python_executable()
CMake functions.  This is useful for dependencies on packages from PyPI.

At the moment this code only handles architecture-independent pure-Python
packages.  It shouldn't be too hard to extend this to handle more complex
wheels, but for now I only need to use it for some pure-Python wheels and so I
haven't tested with more complex wheel files.

This also includes two new manifests for python-six and python-toml that take
use this new builder.

Reviewed By: wez

Differential Revision: D17401216

fbshipit-source-id: d6f74565887c3f004e1c06503dc9ec81599dd697