Handle `File > New > Project from existing sources` more gracefully

[pq]

Follow-up from #530.

When opened via File > New > Project from existing sources, existing Flutter projects get coerced into being Android projects (with just an android folder content root --- no Dart sources). 😦

Getting this working likely involves hooking project import via one of the project import extension points.

This action is generally designed to support importing from other project definitions such as eclipse or gradle and we'd fall under "unspecified origin".

ideally we should handle this case a bit better. In the meantime, @mit-mit can you document this as a known issue?

[pq]

Actually this is a duplicate of #316. Closing in favor of that one.