lib/flakes: move eachSystem from flake-utils

[Artturin]

Description of changes

moved https://github.com/numtide/flake-utils/blob/a4b154ebbdc88c8498a5c7b01589addc9e9cb678/default.nix#L75 to nixpkgs

numtide/flake-utils#62
supersedes #123117

see @lilyball comment #123117 (comment)

intentionally not added to lib to keep it only available at lib.flakes.eachSystem

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/)
• 22.05 Release Notes (or backporting 21.11 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
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[colemickens]

random thought, what if there was another repo for experimental flake lib functions, that was then pulled into nixpkgs-lib in an experimental namespace?

That way we still have a single-lib-util-repo, but we can keep the experimental churn out of nixpkgs proper?

[Artturin]

random thought, what if there was another repo for experimental flake lib functions, that was then pulled into nixpkgs-lib in an experimental namespace?

That way we still have a single-lib-util-repo, but we can keep the experimental churn out of nixpkgs proper?

nix-community/nixpkgs.lib#2

[Artturin]

created https://github.com/nix-community/lib-aggregate

[lilyball]

I don't see how this is any better than just using flake-utils directly. The whole point of the request was that eachSystem is such an incredibly common thing to need that nixpkgs really should provide it directly (especially since eachDefaultSystem's definition is based on what hydra supports for nixpkgs). Writing lib-aggregate.lib.flake-utils.eachSystem really doesn't help anything.