improve natural_path usage in two places

Fixed loader strategy pathing issues where eager loaders such as
:func:`_orm.joinedload` / :func:`_orm.selectinload` would fail to traverse
fully for many-levels deep following a load that had a
:func:`_orm.with_polymorphic` or similar construct as an interim member.

Here we can take advantage of 2.0's refactoring of strategy_options
to identify the "chop_path" concept can be simplified to work
with "natural" paths alone.

In addition, identified existing
logic in PropRegistry that works fine, but needed the "is_unnatural"
attribute to be more accurate for a given path, so we set that
up front to True if the ancestor is_unnatural.

Fixes: #9715
Change-Id: Ie6b3f55b6a23d0d32628afd22437094263745114

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