When the extjson package was merged into the bson package in GODRIVER-328, we did not update the functions to use the bson.Document type. Since we return a bson.Document type in several areas, we should be able to remove the usage of the bson/builder package since this was used for historical purposes (i.e. we did not have the bson.Document type when we built the extjson package but we did have the bson/builder package).