If SERVER-114434 merges, it will be possible for a `QuerySolutionNode` tree to reference multiple collections, but Stage Builders for deduplication steps currently assume that documents come exclusively from the query's "main" collection.

For now, that assumption remains true (even with SERVER-114434 in place), but we'll need to revisit if we ever start constructing plans that have more complicated data access paths over the "foreign" collections in `$lookup`s.