Move the generation of output column names earlier, to right after

Move the generation of output column names earlier, to right after
name resolution and before query transformations such as flattening.
This prevents the names from getting mangled by query transformations,
and obviates hacks in the query flattener that attempt to work around
the name mangling. The resulting code is smaller and faster and gives
more consistent output. Fix to ticket [de3403bf5ae5f72ed].

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