Get rid of almost all direct config access in python-building code.
Review Request #1954 - Created March 20, 2015 and submitted
- Instead, relies more heavily on the existing PythonSetup class,
and a new sibling called PythonRepos.
- These are basically proto-subsystems, and we'll retrofit them
to be part of the subsystem mechanism once we invent that.
- Removes some unnecessary degrees of freedom in config.
For example, there's no reason to allow names of working and scratch dirs
to be configurable.
- Cleans up and streamlines the code.
- Moves some standalone functions into classes, for a more natural design.
- Refactors a test for better code reuse.
CI passes: https://travis-ci.org/pantsbuild/pants/builds/55115225
Tried various manual python builds.
Revision 2 (+203 -219)