top-level/packages-config.nix: Correctly index package subsets delimited by dots

[adisbladis]

Motivation for this change

Fixes the issue discovered by @Atemu in #110348 (comment) where dot delimited subsets were not taken into account.

Note: I haven't tested how this interacts with search at all, I've just assumed it's probably fine. This needs more testing to see what the implications are, hence I've marked it as draft.

cc @garbas

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[Atemu]

I haven't tested how this interacts with search at all

Search have their OOT version of this file to generate the search index; that's where I copied it from actually ;)

[Atemu]

Aborts with infinite recursion for me.

Also needs a bit of formatting cleanup and/or refactoring I think ;)

[adisbladis]

Closing this one as I think #113713 is clearly better.