pkgs/by-name: nested by-name for package sets proof of concept

[quantenzitrone]

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

[Sigmanificient]

While this seems better to me than #483128, i am still more fan of the first proposed solution with pkgs/sets to be honest.

My main issue with this approach it that it break my expectation of pkgs/by-name with the introduction of nesting. Having conditional behavior with if lib.pathIsRegularFile seems like a bit wanky, and it would be solved by having a simple of pkgs/sets grouping everything together to in a flat structure.

I am also concern about the possibility of some sets being hidden from simple github navigation, as it would be the case with lixPackageSets, since it end up at the end of pkgs/by-name/li which is already truncated.