remove should_nest behavior for contains_eager()

remove should_nest behavior for contains_eager()

Fixed regression for 1.4 in :func:`_orm.contains_eager` where the "wrap in
subquery" logic of :func:`_orm.joinedload` would be inadvertently triggered
for use of the :func:`_orm.contains_eager` function with similar statements
(e.g. those that use ``distinct()``, ``limit()`` or ``offset()``). This is
not appropriate for :func:`_orm.contains_eager` which has always had the
contract that the user-defined SQL statement is unmodified with the
exception of adding the appropriate columns.

Also includes an adjustment to the assertion in Label._make_proxy()
which was there to prevent a fixed label name from being anonymized;
if the label is already anonymous, the change should proceed.
This logic was being hit before the contains_eager behavior was
adjusted. With the adjustment, this code is not used.

Fixes: #8569
Change-Id: I161e65041c0162fd2b83cbef40f57a50fcfaf0fd
(cherry picked from commit 57b400f07951f0ae8651ca38338ec5be1d222c7e)