This is very clever, and I wish I had figured this out when I ran into the same problem last summer. A little "smelly" but better than being forced to publically export implementation details.

That said, to play devil's argument, why not just make all implementation details public? I will say I remember being pissed off by Go packages that don't publicize all of their types and interfaces, which made writing tests against their behavior (usually requiring mocking, hence the need for the publicized types/interfaces) to be painful/impossible.