Fix conversion methods to use correct external type

[tchssk]

#3605

It seems that Goify() is also applied to external types and their names change.
I implemented using struct:type:name, but there may be a more better way.

[raphael]

This approach looks good to me, it's consistent with how we've used the struct:type:name to coerce the generated type in other places. Also it's low risk since there should be no other generated code that manipulates the external type. Thank you for the fix!