mixin module template -> undefined identifier

[Dicebot]

By spec name resolution for templates happens in declaration 
scope. Mixin templates are only exception.

If referencing symbol from other module during code gen is a 
unavoidable necessity, you should use instrospection on `T` and 
add its module into code gen as template-local import (something 
like 
https://github.com/rejectedsoftware/vibe.d/blob/master/source/vibe/http/restutil.d#L341)