fixes for usage of the null() and similar constants

Adjusted the "column disambiguation" logic that's new in 1.4, where the
same expression repeated gets an "extra anonymous" label, so that the logic
more aggressively deduplicates those labels when the repeated element
is the same Python expression object each time, as occurs in cases like
when using "singleton" values like :func:`_sql.null`.  This is based on
the observation that at least some databases (e.g. MySQL, but not SQLite)
will raise an error if the same label is repeated inside of a subquery.

Related to :ticket:`7153`, fixed an issue where result column lookups
would fail for "adapted" SELECT statements that selected for
"constant" value expressions most typically the NULL expression,
as would occur in such places as joined eager loading in conjunction
with limit/offset.  This was overall a regression due to issue
:ticket:`6259` which removed all "adaption" for constants like NULL,
"true", and "false", but this broke the case where the same adaption
logic were used to match the constant to a labeled expression referring
to the constant in a subquery.

Fixes: #7153
Fixes: #7154
Change-Id: I43823343721b9e70524ea3f5e8f39dd543a3e92b

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