Split `ABCHierarchyIterator::get_alembic_parent()` into two functions:
- For a given export path, find the Alembic object
- Ensure that that object is usable as parent object (Alembic uses a
specific 'top' object as parent to indicate "no parent").
The new function is `public` as it will be used in an upcoming feature,
and is required to be public then.
No functional changes.
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