Python: buildEnv does not create symlinks to std library modules

[FRidh]

Issue description

While python.buildEnv correctly adds symbolic links to extra packages that are added with extraLibs, it doesn't actually add any symbolic links for the modules that were separated from the interpreter (python 2.7).

Steps to reproduce

$ nix-shell -E 'with import <nixpkgs> {}; with pkgs.pythonPackages; (python.withPackages (ps: with ps; [curses])).env' --run "python -c 'import curses'"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/nix/store/0jmc177zpy0wxqj852vcpapcg1ahk61v-python-2.7.11-env/lib/python2.7/curses/__init__.py", line 15, in <module>
    from _curses import *
ImportError: No module named _curses

I guess we need to set passthru.pythonPath = [];.

cc @domenkozar @bennofs