Show only packages/libraries where match can be found

[patricoferris]

This was suggested on the Coq forum as it relates to Sherlocoq. Copying the user's request here:

Say I’m looking for projects using the “paco” library for instance: currently it returns 10.5k results that are quite hard to browse. Is there any chance to be able to only display the comparatively very short list of opam repos that returned at least one result?

The original discussion thread can be found here: https://coq.discourse.group/t/sherlocoq-search-released-coq-source-code/1984

This is probably particularly more relevant in Coq programs given they have explicit Require/Import statements I think. But it might be useful in OCaml code too :))