Python: add sitecustomize.py, listen to NIX_PYTHONPATH

This commit adds a Nix-specific module that recursively adds paths that
are on `NIX_PYTHONPATH` to `sys.path`. In order to process possible
`.pth` files `site.addsitedir` is used.

The paths listed in `PYTHONPATH` are added to `sys.path` afterwards, but
they will be added before the entries we add here and thus take
precedence.

The reason for adding support for this environment variable is that we
can set it in a wrapper without breaking support for `PYTHONPATH`.

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

@@ -221,6 +221,9 @@ in with passthru; stdenv.mkDerivation {
     find $out/lib/python*/config-* -type f -print -exec nuke-refs -e $out '{}' +
     find $out/lib -name '_sysconfigdata*.py*' -print -exec nuke-refs -e $out '{}' +
 
+    # Include a sitecustomize.py file
+    cp ${../sitecustomize.py} $out/${sitePackages}/sitecustomize.py
+
     # Determinism: rebuild all bytecode
     # We exclude lib2to3 because that's Python 2 code which fails
     # We rebuild three times, once for each optimization level