Matches filepaths to pip package.

Example usage:

$ pip3 install --user requests
$ # Absolute path.
$ pip_file_lookup.py ~/.local/lib/python3.6/site-packages/requests/__init__.py
requests
$ pip3 install --user six
$ # Relative path.
$ pip_file_lookup.py .local/lib/python3.6/site-packages/six.py
six