distutils-r1.eclass: Reuse python_setup for common phases

distutils-r1.eclass: Reuse python_setup for common phases

Rewrite the python_*_all() phase running code to reuse python_setup
instead of hacking on top of python_foreach_impl. The resulting code
is a bit simpler but most importantly, it avoids duplication of code
from python-r1 and ensures that distutils-r1 common phases are directly
altered by changes in python_setup.

The code still needs to reimplement some of the internals. However, it
is mostly limited to code specific to distutils-r1, and should be more
maintainable.