how to use url-for with Engines?

I created a demo repo that uses `{{url-for "an-external-route"}}`, but I get "There is no route named..."

Where `"an-external-route"` is a mapping of:

```
externalRoutes: {
'an-external-route': 'a-parent-app-route.index'
}
```

If I change `"an-external-route"` to be the name of an _actual route_ in the host app, then it works.

https://github.com/amk221/-ember-url-for-helper-example

Do we need `url-for-external`?