Establish that contains_eager()->alias can be replaced by of_type

One test in test_of_type was creating a cartesian product
because contains_eager() was used with "alias" to refer
to a with_polymorphic(), but the wp was not used with of_type(),
so the pathing did not know that additional entities were present.

while the docs indicate that of_type() should be used, there is no
reason to use "alias" when you are using of_type().   Attempts
to make this automatic don't work as the current usage contract
with "alias" is that the contains_eager() chain can continue
along in terms of the base entities, which is another example
of the implicit swapping of entities for an aliased version of
themselves that really should be entirely marked as deprecated
throughout 1.4 and removed in 2.0.

So instead, add test coverage for the of_type() versions of
things and begin to make the case that we can remove "alias"
entirely, where previously we thought we would only deprecate
the string form.

Fixes: #5096
Change-Id: Ia7b021c4044332ab3282267815f208da64410e95

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