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.