Blender Git Loki

Cleanup: USD, refactored export graph construction

The construction of the export graph is now split into a few steps:
- Construct a `HierarchyContext` object for the to-be-exported object.
- Determine the graph index, i.e. the export-parent and the duplicator.
- Update the `HierarchyContext` object for this graph index.

This allows the upcoming new Alembic exporter to override the location of
an object in the export graph, in order to support "flattened" exports.
Aside from that, this also simply makes the code cleaner.

No functional changes.