patman: rename main script to __main__.py

patman: rename main script to __main__.py

This allows running the package as a Python module, like e.g.:

    $ python -m patman

It also prevents Pytest from attempting to parse main.py, which
would cause errors.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Maxim Cournoyer <maxim.cournoyer@savoirfairelinux.com>
Fix up main.py in __init__.py:
Signed-off-by: Simon Glass <sjg@chromium.org>