feat: add convenience eachDefaultSystemMap

[Misterio77]

Hello!

As mentioned in #59, eachSystemMap is really powerful now (and helps avoid using //).

It would be nice if we had a eachDefaultSystemMap = eachSystemMap defaultSystems, so we don't have to let it in every flake. So here it is.

For reference, here's an example flake that outputs a module and a package using eachDefaultSystem (with the older defaultPackage and co):

{
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
    utils.url = "github:numtide/flake-utils";
  };

  outputs = { self, nixpkgs, utils }:
    rec {
      nixosModules.example = import ./module.nix;
      nixosModule = nixosModules.example;
    } // (utils.lib.eachDefaultSystem (system: rec {
      packages.example = (import nixpkgs { inherit system; }).callPackage ./default.nix { };
      defaultPackage = packages.example;
    }));
}

And using eachDefaultSystemMap (with the newer *.default attribute):

{
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
    utils.url = "github:misterio77/flake-utils/each-default-system-map";
  };

  outputs = { self, nixpkgs, utils }: {
    nixosModules = rec {
      example = import ./module.nix;
      default = example;
    };

    packages = utils.lib.eachDefaultSystemMap (system: rec {
      example = (import nixpkgs { inherit system; }).callPackage ./default.nix { };
      default = example;
    });
  };
}

Thanks!

[Mic92]

I think the function makes sense. @zimbatm what is your thought?

[aakropotkin]

Document it front and center in the README and merge this bad boy.
I added a second example to my original issue that conditionally adds defaultPackage for Nix versions < 2.7.

[zimbatm]

Does anybody want to send a docs PR? Please 🙏