Cannot lookup scoped packages

[buschtoens]

Causes ember-engines/ember-engines#524

As per ember-engines/ember-engines#524 (comment):

We can probably teach the resolver specifically about :@, which should unblock us here...

I'm starting a PR for this.

/cc @rwjblue @dfreeman @lennyburdette @gossi

[buschtoens]

I've linked #270 in our production app, which is using lazy-loaded, scoped engines, and so far it seems to work nicely.

[buschtoens]

Closed via #330.