FR: battery for flake-parts independent system access

[vic]

We currently have flake-parts inputs' and self' support.

I wrote a tiny system' function, used like this

vic.everywhere.nixos =
    { pkgs, system', ... }:
    {
      users.users.vic.packages = [ (system' inputs.helium).packages.helium ];
    };

maybe that can be useful to others now that Den explicitly supports non-flakes and non-flake-parts.