Documents the reason why it's needed and also prevents the
ensureNewerSourcesHook call being evaluated again and again for every
single Python package.
Python libraries or modules now have an attribute `pythonModule = interpreter;` to indicate
they provide Python modules for the specified `interpreter`.
The package set provides the following helper functions:
- hasPythonModule: Check whether a derivation provides a Python module.
- requiredPythonModules: Recurse into a list of Python modules, returning all Python modules that are required.
- makePythonPath: Create a PYTHONPATH from a list of Python modules.
Also included in this commit is:
- disabledIf: Helper function for disabling non-buildPythonPackage functions.
