In SERVER-104520, we've moved the classic engine code to a separate folder to be more consistent with the filesystem structure of SBE.

As a follow up, it'd be nice to also create a Bazel library for the classic engine (the classic sources currently link directly with the query_exec library), and also its own unit test. Besides, from a cursory look at the code, I expect it to be possible to split plan_stats.h into a part that's common to both SBE and classic, and another which is specific only to classic.

The second proposal has the added benefit of benefiting compilation times.