Tags discoverability

[rjbou]

It would be nice to have a way to search exclusively by tag (currently the tag search is a string search on all metadata), or a tag cloud to discover some packages.
First reported in ocaml/opam#6842
related to #82 and #134

[rjbou]

Discussed in 2026-01-19 meeting: It is definitively a good to have. If someone want to add this feature, it'll be very welcome!