swh-scheduler will deduplicate listed origins according to their URL
and visit type but not according to their extra loader arguments.
Previously, listed origins were yielded after each processed artifact
in a page so we could lose some package version info due to the
deduplication process.
So ensure to yield listed origins once all artifacts in a page have
been processed.
Previously, the run method was returning the total count of ListedOrigin
objects sent to scheduler database.
However, some listers can send multiple ListedOrigin objects for a given
origin URL during the listing process, for instance when an origin is
contained in multiple pages (e.g. gogs listing) or when the listing
is gathering multiple versions of an origin spread across multiple
pages (e.g. maven listing).
This changes ensures an accurate count of listed origins by maintaining
a set of origin URLs associated to the sent ListedOrigin objects.
