UsdImagingRerootingSceneIndex's rerooting data sources are not re-usable/exposed

[debloip-adsk]

In rerootingSceneIndex.cpp, the _RerootingSceneIndexContainerDataSource, _RerootingSceneIndexPathArrayDataSource and _RerootingSceneIndexPathDataSource are located within an anonymous namespace, and are not accessible from outside the file.

Given the usefulness of these data sources, we ended up copying these over in our project to leverage the rerooting behavior without having to use a whole scene index, but it would be more convenient to have the original data sources exposed, to promote reusability and also benefit from any code changes these may receive.

[jesschimein]

Filed as internal issue #USD-10660

(This is an automated message. See here for more information.)