pattern: Use URL network location as instance name when not provided

Make the instance parameter of the base pattern lister optional and set
lister name to URL network location when not provided.

It simplifies lister creation when associated forge type have a lot of
instances in the wild (e.g. gitlab or cgit) while giving more details
about the listed forge instance.

Also process listers for forge with multiple instances (cgit, gitea,
gitlab, phabricator and tuleap) to ensure URL network location will be
used when instance parameter is not provided.

Related to T3403