Add Gitweb lister

Depending on some instances, we have some specific heuristics, some instances:
- have summary pages which do not not list metadata_url (so some
  computation happens to list git:// origins which are cloneable)
- have summary page which reference metadata_url as a multiple comma separated urls
- lists relative urls of the repository so we need to join it with the main instance url
  to have a complete cloneable origins (or summary page)
- lists "down" http origins (cloning those won't work) so lists those as cloneable https
  ones (when the main url is behind https).

Refs. swh/devel/swh-lister#1800