default.nix

{ lib, ... }: with lib; {

  config = {
    fileSystems."/".device = "/dev/sda";
    boot.loader.grub.device = "nodev";
  };

  options.fileSystems = mkOption {
    type = types.loaOf (types.submodule ({ name, ... }: {
      config = mkIf (name == "/foo") {
        options = [ "foo" ];
      };
    }));
  };
}

Is equivalent to:

{ lib, ... }: with lib; {

  imports = [ ./foo.nix ];

  fileSystems."/".device = "/dev/sda";
  boot.loader.grub.device = "nodev";
}

With foo.nix containing:

{ lib, ... }: with lib;
{
  options.fileSystems = mkOption {
    type = types.loaOf (types.submodule ({ name, ... }: {
      config = mkIf (name == "/foo") {
        options = [ "foo" ];
      };
    }));
  };
}