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/pypy/2.7/default.nix

@@ -137,7 +137,7 @@ in stdenv.mkDerivation rec {
     inherit zlibSupport libPrefix sitePackages;
     executable = "pypy";
     isPypy = true;
-    buildEnv = callPackage ../../wrapper.nix { python = self; };
+    buildEnv = callPackage ../../wrapper.nix { python = self; inherit (pythonPackages) requiredPythonModules; };
     interpreter = "${self}/bin/${executable}";
     withPackages = import ../../with-packages.nix { inherit buildEnv pythonPackages;};
     pkgs = pythonPackages;