Fix swh-scanner for python > 3.7

!79 (closed) introduce a couple of problems:

lack the dependency on https://pypi.org/project/importlib-metadata/ (should be needed only for python 3.7)
the package name is "swh.scanner", not "swh-scanner", in https://forge.softwareheritage.org/source/swh-scanner/browse/master/swh/scanner/cli.py$73 ; using "swh-scanner" seems to work for importlib_metadata, but not with stdlib's importlib.metadata (in python >= 3.8), so it's probably better to use 'swh.scanner' there.

Migrated from T3209 (view on Phabricator)