PythonDependency: massive refactoring

Kill PythonHelper and cut everything out of PythonDependency beyond
setting up PYTHONPATH and supporting the existing DSL without changes.

This isn't ready to be merged yet, it's up for review of the approach.
I next to test a lot more formulae, Python 3 support and modify some to
behave nicely. Basic tests seem to show this works, though.