Improve code for queries of unavailable packages
Exclude installed depopts of dependency queries
Cleaner interface to query (reverse) package dependencies

Now consistent with the other `opam list` commands, and with a better
structured argument set.
Thanks @dbuenzli for the good parameter names suggestion.
Merge pull request #1498 from OCamlPro/better-deps-queries

Add `opam list --[rec-]required-by | --[rec-]depends-on` to query dependencies