against a non-selectable, such as a :func:`.literal_column`, and then
an attempt was made to use :meth:`.Query.join` such that the "left"
side would be determined as ``None`` and then fail.  This condition
is now detected explicitly.

target of :paramref:`.relationship.secondary` for the purposes
of creating very complex :func:`.relationship` join conditions.
The change includes adjustments to query joining, joined eager loading
to not render a SELECT subquery, changes to lazy loading such that
the "secondary" target is properly included in the SELECT, and
changes to declarative to better support specification of a
join() object with classes as targets.

alias when saying query(B).join(B.cs), where "C" is a joined inh
class; however, this implicit alias was created only considering
the immediate left side, and not a longer chain of joins along different
joined-inh subclasses of the same base.   As long as we're still
implicitly aliasing in this case, the behavior is dialed back a bit
so that it will alias the right side in a wider variety of cases.
[ticket:2903]

adapter to an aliased-mapped, non-polymorphic selectable that prevented us from referring
directly to that selectable.

- clean up inspect() calls within query._join()
- make sure join.alias(flat) propagates
- fix almost all assertion tests

method has been turned off.  The specific behavior is now
availble via a new method :class:`.Query.select_entity_from`.
[ticket:2736]

- went through examples/ and cleaned out excess list() calls

fundamental and general purpose heuristic.   this initial approach
has about 60 tests failing but seems to have gone pretty far