PythonInterpreter should not need to be manually setup with vendored extras

[jsirois]

This shows up as a wart when pex is used as a library. Folks have to do this:

import pex
from pex.interpreter import PythonInterpreter


interpreter = pex.vendor.setup_interpreter(PythonInterpreter.get())

The vendor package is really not meant for public consumption, but populating interpreters with appropriate extras (setuptools & wheel) that pex has vendored for use with PEXBuilder should be automatic.