here's the flat join thing.  it just works.   Changing the existing compiled SQL assertions
might even be most of the tests we need (though dedicated sql tests would be needed anyway)
- eager loads
- two suite of SQL assertions converted
- improve overlapping selectables, apply to both query and relationship
- clean up inspect() calls within query._join()
- make sure join.alias(flat) propagates
- fix almost all assertion tests
- add a flag to DefaultDialect for this so that people will have some
workaround
clean up the dialect selection thing here
repair these tests now that we allow join from selectable to fromgrouping