can anyone please help me writing a module for a system config? I am trying to add a simple module like this:
```nix
{ pkgs, lib, ... }:
with lib;
{
  options = {
    _ = {
      username = mkOption {
        type = types.str;
        default = "cab";
      };
      hostname = mkOption {
        type = types.str;
        default = "yuna";
      };
    };
  };
  config = {};
}
```
but I am getting this error:
```
error: in 'toFile': the file 'options.xml' cannot refer to derivation outputs, at /nix/store/rfk5j99kh4ny0jfi0m88yapv3pbw19nr-nixos-20.03.1619.ab3adfe1c76/nixos/nixos/lib/make-options-doc/default.nix:89:16
```
I tried moving that module to nix store (via toFile readFile) but it didn't help.