maven: Create one origin per package instead of one per package version
Previously the maven lister was creating an origin for each source archive (jar, zip) it discovered during the listing process.
This is not the way Software Heritage decided to archive sources coming from package managers. Instead one origin should be created per package and all its versions should be found as releases in the snapshot produced by the package loader.
So modify the maven lister in order to create one origin per package grouping all its versions.
This change also modifies the way incremental listing is handled, ListedOrigin instances will be yielded only if we discovered new versions of a package since the last listing.
Tests have been updated to reflect these changes.
Related to T3874
Migrated from D7710 (view on Phabricator)