Index and show `pkgConfigModules`

[roberth]

We've started cataloging pkg-config packages ("modules"). It'd be awesome to have this data in the package search. The format is quite simple

    # check-meta.nix
    pkgConfigModules = listOf str;

The set of modules can vary from platform to platform, but usually it's a constant set, so you could cut that corner if that makes for an easier start.

See discourse post

[ncfavier]

Seems rather specific, I don't have motivation to do this but feel free to open a PR I guess. At least for the indexing part; I'm really not sure we need to show these to users.

Also if this is just supposed to reflect the files in /lib/pkgconfig that sounds like a job for nix-index, not another metadata field?

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2023-07-20-documentation-team-meeting-notes-65/30876/1