Namespace package handling

[BenMatase]

I've added the changes mostly from https://github.com/dillon-giacoppo/rules_python_external/blame/master/extract_wheels/lib/namespace_pkgs.py so that these rules can correctly handle namespace packages. Since the source code is essentially copied, I've added an Apache license header to the source code file added.

I've added a test using two azure-mgmt packages to confirm that this functionality.

[BenMatase]

This would solve https://github.com/apt-itude/rules_pip/issues/46