Blender Git Loki

Cleanup: Alembic export, split `ABCHierarchyIterator::get_alembic_parent()`

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.