message: Fix namespaced struct/field name collision detection
  Changes the use of `LookupStruct` to `LookupCreateStruct` in
  `ParseField` to also detect when collisions happen in namespaces.
diff: @@ -677,7 +677,7 @@ CheckedError Parser::AddField(StructDef &struct_def, const std::string &name,
  CheckedError Parser::ParseField(StructDef &struct_def) {
  std::string name = attribute_;
  - if (LookupStruct(name))
  + if (LookupCreateStruct(name, false, false))
  return Error("field name can not be the same as table/struct name");
  std::vector<std::string> dc = doc_comment_;
  