Python: the pythonModule attribute

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.

pkgs/development/interpreters/python/wrapper.nix

@@ -2,13 +2,14 @@
 , extraLibs ? []
 , extraOutputsToInstall ? []
 , postBuild ? ""
-, ignoreCollisions ? false }:
+, ignoreCollisions ? false
+, requiredPythonModules
+, }:
 
 # Create a python executable that knows about additional packages.
 let
-  recursivePthLoader = import ../../python-modules/recursive-pth-loader/default.nix { stdenv = stdenv; python = python; };
   env = let
-    paths = stdenv.lib.closePropagation (extraLibs ++ [ python recursivePthLoader ] ) ;
+    paths = requiredPythonModules (extraLibs ++ [ python ] ) ;
   in buildEnv {
     name = "${python.name}-env";