pyside: fix on Python 3.x, closes #25328

Pyside requires several tools that do not provide Python modules. They
therefore do not need to be build Python-version dependent and so we
move them out of `python-packages.nix`.

Furthermore, shiboken needs libxml2 and libxslt libraries but not their
Python bindings.

pkgs/development/python-modules/pyside/default.nix

@@ -1,5 +1,6 @@
 { lib, fetchurl, cmake, python, buildPythonPackage, pysideGeneratorrunner, pysideShiboken, qt4 }:
 
+# This derivation provides a Python module and should therefore be called via `python-packages.nix`.
 buildPythonPackage rec {
   name = "pyside-${version}";
   version = "1.2.4";