_mapper_zero() does not return a mapper in all cases

[Mike Bayer]

select_from_entity is a class or AliasedClass. change the name most likely, as returning an entity as it does now preserves more information.

[Mike Bayer]

and of course _entity_zero() returns _MapperEntity so there's some name conflict here

[Mike Bayer]

Rename Query._mapper_zero() to Query._entity_zero()

To be more descriptive of the use of _mapper_zero(), rename it to _entity_zero(), but also supply a new _mapper_zero() function that more strictly returns a mapper. The existing _entity_zero() function is renamed to _query_entity_zero. _only_mapper_zero() is removed as it isn't used. Divide up the existing calling functions to refer to the appropriate new method.

Change-Id: I8780c3235e87b4936c6daf64d9d299b22b6e1260 Fixes: #3608

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