Allow join() to pick the best candidate from multiple froms/entities

Allow join() to pick the best candidate from multiple froms/entities

Refactored :meth:`.Query.join` to further clarify the individual components
of structuring the join. This refactor adds the ability for
:meth:`.Query.join` to determine the most appropriate "left" side of the
join when there is more than one element in the FROM list or the query is
against multiple entities.  In particular this targets the regression we
saw in :ticket:`4363` but is also of general use.   The codepaths within
:meth:`.Query.join` are now easier to follow and the error cases are
decided more specifically at an earlier point in the operation.

Fixes: #4365
Change-Id: I403f451243904a020ceab4c3f94bead550c7b2d5