When parsing pom files, we are only interested to extract a VCS URL
(git, hg, svn) in order to create associated loading tasks.
In that case, the groupId and artifactId are not used by the lister
so better removing their extraction, plus it will prevent errors when
those info are missing in pom files.
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
This aligns the behavior with other listers (e.g. sourceforge, ...) to continue listing
if some information is not retrievable at all.
Related to T3874
I would like to use it as the metadata authority URI in the loader,
instead of '{p_url.scheme}://{p_url.netloc}/', which I do not think
is accurate, as it is possible to have multiple Maven instances at
the same netloc.
The Maven lister retrieves the maven central indexes, exports them in a
convenient text format, and parse them to identify all src archives and
pom files in the maven repository. Then the pom files are downloaded and
analysed to find and yield any scm reference.
Note: This is a new version of the maven lister diff D6133 which takes
into account the initial round of reviews.
Related to T1724
