WIP: treewide: splice more nested package sets

[Ericson2314]

Motivation for this change

So far this just has some functions that might come in handy. Next step
is to find package sets that provide things used as non-buildInputs
(most of them seem to not fit that?).

I've always hated this splicing monster. Maybe others can tell me where
this might be worth it.

CC @FRidh @globin

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.

[FRidh]

Perhaps split attr on dots to allow for nested sets?

[FRidh]

I wonder how to access the buildPackages variant of pythonPackages, see b1d8335

[stale]

I marked this as stale due to inactivity. → More info

[Artturin]

replaced by #207652