excise direct Pex imports

[cburroughs]

In at least two places we directly import from Pex:

src/python/pants/core/subsystems/python_bootstrap.py:from pex.variables import Variables
tests/python/pants_test/init/test_plugin_resolver.py:from pex.interpreter import PythonInterpreter

And it is a dependency in ./3rdparty/python/user_reqs.lock

However, the intended stable interface of Pex is the cli instead of using it 'as a library'. Having two different versions of Pex floating around is also a continuing source of confusion.