We have two copies of this code, and the python module one is vastly
superior, not just because it allows choosing which python executable to
base itself on. Unify this. Fixes various issues including non-Windows
support for sysconfig, and pypy edge cases.
In preparation for wholly merging the dependency handling from the
python module into dependencies.*, move the unique class definitions
from there into their new home in dependencies.python, which is
semantically convenient.
In preparation for handling more work inside dependencies.*, we need to
be able to run a PythonExternalProgram from the python dependency. Move
most of the definition -- but only the parts that have no interest in a
ModuleState -- and subclass a bit of sanity checking that we need to
handle specially when used in the module.
Eli Schwartz