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/cpython/2.7/default.nix

@@ -201,7 +201,7 @@ in stdenv.mkDerivation {
     in rec {
       inherit libPrefix sitePackages x11Support hasDistutilsCxxPatch;
       executable = libPrefix;
-      buildEnv = callPackage ../../wrapper.nix { python = self; };
+      buildEnv = callPackage ../../wrapper.nix { python = self; inherit (pythonPackages) requiredPythonModules; };
       withPackages = import ../../with-packages.nix { inherit buildEnv pythonPackages;};
       pkgs = pythonPackages;
       isPy2 = true;

pkgs/development/interpreters/python/cpython/3.4/default.nix

@@ -160,7 +160,7 @@ in stdenv.mkDerivation {
   in rec {
     inherit libPrefix sitePackages x11Support;
     executable = "${libPrefix}m";
-    buildEnv = callPackage ../../wrapper.nix { python = self; };
+    buildEnv = callPackage ../../wrapper.nix { python = self; inherit (pythonPackages) requiredPythonModules; };
     withPackages = import ../../with-packages.nix { inherit buildEnv pythonPackages;};
     pkgs = pythonPackages;
     isPy3 = true;

pkgs/development/interpreters/python/cpython/3.5/default.nix

@@ -154,7 +154,7 @@ in stdenv.mkDerivation {
   in rec {
     inherit libPrefix sitePackages x11Support;
     executable = "${libPrefix}m";
-    buildEnv = callPackage ../../wrapper.nix { python = self; };
+    buildEnv = callPackage ../../wrapper.nix { python = self; inherit (pythonPackages) requiredPythonModules; };
     withPackages = import ../../with-packages.nix { inherit buildEnv pythonPackages;};
     pkgs = pythonPackages;
     isPy3 = true;

pkgs/development/interpreters/python/cpython/3.6/default.nix

@@ -153,7 +153,7 @@ in stdenv.mkDerivation {
   in rec {
     inherit libPrefix sitePackages x11Support;
     executable = "${libPrefix}m";
-    buildEnv = callPackage ../../wrapper.nix { python = self; };
+    buildEnv = callPackage ../../wrapper.nix { python = self; inherit (pythonPackages) requiredPythonModules; };
     withPackages = import ../../with-packages.nix { inherit buildEnv pythonPackages;};
     pkgs = pythonPackages;
     isPy3 = true;