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