The goal of this task is to create a USD importer loosely based on the Alembic importer. This means the following:
- Simple one-time importing (via File → Import → USD).
- Mesh animation loaded via a modifier.
- Transform animation loaded via a constraint.
- Feature parity with the current USD Exporter.
- No support for the collection-based workflow (T68933) yet.
- Separation of the code that translates USD datastructures into Blender datastructures, so that this can be reused by a future collection-based importer.