Similar rationale as SERVER-92832. Let's move the code for the classic and SBE stage builders from db/query into a new subdirectory db/query/stage_builders. As part of this task, we need to assign code ownership for the stage builders. I believe neither the QE nor the QO charters currently mention the stage builders. This code is part of the query compilation pipeline (so one could argue it belongs with QO), but a lot of it is pretty deeply connected to SBE and it has historically been implemented by the QE team. My inclination is therefore to assign ownership of the new directory to the QE team.