Cleanup with query aliasing

Try to simplify some of the "adapter" stuff in query:

1. identify that join(.., aliased=True) doesn't work if the
right side has no mapper.   The adaption of the right side is
done via the mapper with aliased(), so that doesn't effect
a selectable only.  raise an error, so we can simplify
the code.

2. build fewer adapter objects.   these are confusing to follow
and we should try to figure out exactly what purpose which
one serves where and make that clear.

Change-Id: I18dfcd01e6ad533aa0b8d557fc637ee2766ed050

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