Fold entities into existing joins when resolving FROM ambiguity

Fold entities into existing joins when resolving FROM ambiguity

Fixed 1.3 regression in new "ambiguous FROMs" query logic introduced in
:ref:`change_4365` where a :class:`.Query` that explicitly places an entity
in the FROM clause with :meth:`.Query.select_from` and also joins to it
using :meth:`.Query.join` would later cause an "ambiguous FROM" error if
that entity were used in additional joins, as the entity appears twice in
the "from" list of the :class:`.Query`.  The fix resolves this ambiguity by
folding the standalone entity into the join that it's already a part of in
the same way that ultimately happens when the SELECT statement is rendered.

Fixes: #4584
Change-Id: Ic62ca09c6d329695b21ed4f1be8621edcbb18e19