Use faster version of Python 'checker' script

[Chris Warburton]

Nixpkgs runs a script after building a Python package, which seems to
check for problems in the "$out" dir like duplicate module names and
stuff. The problem is, this script gets added to the top-level directory
of the Nix store, since it's referenced as ./the-script.py; this causes
the Python interpreter to look through at every /nix/store/* entry to
see if it's a Python module (for __init__.py and stuff like that). This
is SLOW.

There's a fix at:

https://github.com/NixOS/nixpkgs/commit/249b34aadca7038207492f29142a3456d0cecec3

We should (a) check whether <nixpkgs> has the fix (e.g. getting the path
of the script somehow and seeing if it's in a directory); we should emit
a warning (via trace) if the fix is found, since that indicates that the
patch may now be obsolete (assuming, of course, that we're on our
'normal' <nixpkgs>; we can ignore the warning if we're in some
bleeding-edge environment; hence why it should be a warning rather than
an assertion). Then we should (b) override the implementation in
<nixpkgs> if necessary, to ensure the subdirectory patch is used.