Insert primary entity in dynamic "secondary"

Insert primary entity in dynamic "secondary"

Fixed regression caused by :ticket:`4349` where adding the "secondary"
table to the FROM clause for a dynamic loader would affect the ability of
the :class:`.Query` to make a subsequent join to another entity.   The fix
adds the primary entity as the first element of the FROM list since
:meth:`.Query.join` wants to jump from that.   Version 1.3 will have
a more comprehensive solution to this problem as well (:ticket:`4365`).

Fixes: #4363
Change-Id: I1abbb6207722619dc5369e1fd96de43d60a1ee62
(cherry picked from commit 40c1a46e993b5c5ff917ce41c3dca66c139bde94)