pattern: Ensure accurate origin counts returned by run method

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.

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Migrated from D8565 (view on Phabricator)