PythonDependency: massive refactoring.

Kill PythonHelper and cut everything out of PythonDependency beyond
setting up PYTHONPATH and supporting the existing DSL without changes.
Caveats: add automatic Python caveats.
Remove use of python.standard_caveats.
libxml2: decouple mad Python stuff.
Add helpers to wrap bin files with PYTHONPATH set.
Use new PYTHONPATH wrapper.
Use `system 'python'`.
Remove python import tests from non-python stuff.
Remove uses of python.libdir.