Lookup index columns in parent table by key for copy

Lookup index columns in parent table by key for copy

Fixed regression in 1.2 due to :ticket:`4147` where a :class:`.Table` that
has had some of its indexed columns redefined with new ones, as would occur
when overriding columns during reflection or when using
:paramref:`.Table.extend_existing`, such that the :meth:`.Table.tometadata`
method would fail when attempting to copy those indexes as they still
referred to the replaced column.   The copy logic now accommodates for this
condition.

Change-Id: I521aa2c9f3baa0e84598bbdd6ffe4bf07b6e3ba8
Fixes: #4279