691def10a0 made all Subquery() instances
equal to each other which broke aggregation subquery pushdown which
relied on object equality to determine which alias it should select.
Subquery.__eq__() will be fixed in an another commit but
Query.rewrite_cols() should haved used object identity from the start.
Refs #30727, #30188.
Thanks Makina Corpus for the report.
Subquery annotation references must be resolved if they are excluded
from the GROUP BY clause by a following .values() call.
Regression in fb3f034f1c.
Thanks Makina Corpus for the report.