Fixed #25307 -- Conditional aggregation over subqueries

[newportt]

https://code.djangoproject.com/ticket/25307

[charettes]

I think this could benefit from being split in multiple test methods.

[akaariai]

As said in the ticket, I believe the correct approach is to add get_source_expressions()/set_source_expressions() to WhereNode and Lookup. See #6267.

[newportt]

@akaariai I made some changes in 4063654 to split out the unit tests and add another assertion for the isinstance(self.group_by, list) case. This can be closed in favor of #6267

[akaariai]

I added the tests in a separate commit to give credit for the real author of the tests.

[e78908b943c8]

ValueError: too many values to unpack (expected 2)

This seems to fail with the above exception if multiple conditions are passed to When as such

Sum(Case(When(date__gte=dr.start, date__lt=dr.end, then='price')))

[charettes]

@e78908b943c8, this was superseded by #6267. If it also exhibit the same failures please report it there instead.