Review #2525 for code improvements: ORDER BY resolution in grouped queries #2630

This is not a bug report. It is an invitation to look at #2525 again, because it may treat a symptom rather than the cause. The goal is to understand the change and decide whether anything should be improved.

The concerns, from a quick read of the diff:

• A case-by-case lookup instead of one resolution step. To find out what an ORDER BY column refers to in a grouped query, compileSelectGroup2 gets two more ad-hoc maps (grouped column to its alias, projected names) and two more branches.
• ORDER BY items are not resolved in one place. The $$$n sort keys are computed separately for plain queries, for grouped queries (here), and twice for set operations in queryfn3 (see Keep ORDER BY / LIMIT of the first SELECT in a set operation on every run to close #2605 #2608, Remove order keys before comparing rows in a set operation to close #2610 #2611). One step that maps each ORDER BY item to an output column, an alias or a group key before compiling might replace all of them.