Move py.typed from swh/loader/{package,core}/ to swh/loader/
There is code in swh/loader/cli.py, and swh-loader-metadata will need to import cli.py, causing mypy to complain when py.typed is missing.
Staging instance, all changes can be removed at any time
There is code in swh/loader/cli.py, and swh-loader-metadata will need to import cli.py, causing mypy to complain when py.typed is missing.