SlotBasedStageBuilder currently assumes that the entire plan refers to a single collection, and only that collection reference is used when building scans.

This assumption is no longer true, when adding support to $lookup, as the $lookup references foreign collections.

We need to modify SlotBaseStageBuilder to provide access to all referenced collections. Possibly by using:
map of string -> CollectionPtr _collections.
in the StageBuilder instead of just:
const CollectionPtr& _collection