make doctor only inspects site-packages, not PYTHONPATH entries or the working directory #1267

Where: scripts/check_environment.py.

Current gap: The check looks at the purelib, platlib and user site directories. A stdlib-named module in a directory listed in PYTHONPATH, or a stray random.py next to the script being run, shadows the standard library just as well but is not reported.

Suggested approach: Also scan the entries of sys.path that come from PYTHONPATH and the current directory, and name where each came from.

Done when: a test with a fake PYTHONPATH directory containing enum.py reports it.