render col name in on conflict set clause, not given key

render col name in on conflict set clause, not given key

Fixed bug where the PostgreSQL :meth:`_postgresql.Insert.on_conflict`
method and the SQLite :meth:`_sqlite.Insert.on_conflict` method would both
fail to correctly accommodate a column with a separate ".key" when
specifying the column using its key name in the dictionary passed to
``set_``, as well as if the :attr:`_sqlite.Insert.excluded` or
:attr:`_postgresql.Insert.excluded` collection were used as the dictionary
directly.

Fixes: #8014
Change-Id: I67226aeedcb2c683e22405af64720cc1f990f274
(cherry picked from commit 927abc3b33f10464ed04db3d7a454faeb6e729f2)