Refactor stubgen to do semantic analysis #3982

Currently stubgen just builds an AST and works mostly at a syntactic level. Given that, most type annotations during its executions are actually instances of UnboundType and forces stubgen to make some assumptions because of lack of information (for example in a type X.Y.Z, which part of that path is a module name, or if actually X = Y is a type alias or not)

This comes from #3169 (comment)