Don't assume key when matching cloned columns in _make_proxy

Don't assume key when matching cloned columns in _make_proxy

Fixed issue where internal cloning of SELECT constructs could lead to a key
error if the copy of the SELECT changed its state such that its list of
columns changed.  This was observed to be occurring in some ORM scenarios
which may be unique to 1.3 and above, so is partially a regression fix.

For 1.4, the _is_clone_of key will be removed entirely as it seems to
have no purpose.  This commit is the initial backport to 1.3 which
includes tests.

Fixes: #4780
Change-Id: I0c64962a2eba3763bea3107fc7c7d7aed8244430