[Distutils] fix setup_requires by allowing it in setup.cfg or a setup_requires.txt

[Daniel Holth]

Come to think of it it would be pretty easy to implement this feature
without involving pip at all by writing a small setup.py replacement
that knows how to look for and install the aforementioned
requirements. If it does not finds them then it shells out to pip to
install them in a build-dep directory. It adds build-dep to sys.path,
and runs real-setup.py in its own process.