consider column.name directly when evaluating use_existing_column

consider column.name directly when evaluating use_existing_column

Fixed issue where new :paramref:`_orm.mapped_column.use_existing_column`
feature would not work if the two same-named columns were mapped under
attribute names that were differently-named from the explicit name given to
the column itself. The attribute names can now be differently named when
using this parameter.

Fixes: #9332
Change-Id: I43716b8ca2b089e54a2b078db28b6c4770468bdd