core/lib/lib/tests/modules/submoduleFiles.nix

{ lib, ... }: {
  options.submodule = lib.mkOption {
    default = {};
    type = lib.types.submoduleWith {
      modules = [ ({ options, ... }: {
        options.value = lib.mkOption {};

        options.internalFiles = lib.mkOption {
          default = options.value.files;
        };
      })];
    };
  };

  imports = [
    {
      _file = "the-file.nix";
      submodule.value = 10;
    }
  ];
}