It’s in base.rb in the same directory. And it’s doing exactly what the name of the function suggests, it’s looking for the topic_id by finding the import_id (which I assume is the message ID in this case} in a topic custom field (or maybe a post custom field?).
That’s better than the ones that take a week. 
(Sometimes you can do stuff to have the import script import only the stuff you’re trying to debug; figuring out how to do so is left as an exercise to the reader.)
You can try looking at the database and see if the parent message is getting imported and if it has a import_id topic/post custom field.