The code for the query planner currently lives in db/query/ alongside a bunch of other query code. I propose that we introduce a new directory db/query/planner/ to house the code for the query planner itself.

I think the model should be that files in db/query/ are owned by the entire query team ("@10gen/query"). Sub-components that should have more specific ownership will then live in subdirectories – we already have subdirectories for ce/ and boolean_simplification/ and so-on. So planner/ would just be another subdirectory fully owned by the QO team.