SERVER-101271 solved a bug that was caused by misunderstanding the fieldSet variable in bucket unpacker and bucket spec. We have many variables in these two classes that define the unpacker behavior, but managing and understanding all of these variables is confusing and leads to bug. For example we use a combination of _includeMetaField and unpacker behavior to determine if we removed the metaField from the original field set to decide if we internalized a $project, which is not obvious from the code. 

This ticket should clarify behavior of variables in these two classes and potentially see if some can be removed to simplify the code.