lib/system: move toLosslessStringMaybe into lib/tests

[ghost]

Description of changes

toLosslessStringMaybe is not used by anything other than lib/tests, so it can be private to that file.

I don't think this function was terribly well thought-through.

1. If people start using it, we will become permanently dependent on the ability to test platforms for equality.
2. It makes the elaboration process more fragile, because it encourages code outside of nixpkgs to become sensitive to the minute details of how elaboration happens.
3. It turns platforms for which toLosslessStringMaybe plat == null into second-class citizens. Our long term direction should be away from assuming that a platform is just a string -- places where we make that assumption already cause massive headaches for pkgsStatic and pkgsLLVM whose stdenv.hostPlatforms both fall into this category.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.11 Release Notes (or backporting 23.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[alyssais]

Should we do a deprecation period for lib.systems.toLosslessStringMaybe? It might have out-of-tree users.

[ghost]

Should we do a deprecation period for lib.systems.toLosslessStringMaybe? It might have out-of-tree users.

It was added just last week in 5ff6f51