Reduce the number of possible locations used to find libraries,
to simplify troubleshooting.
Only keep '*_ROOT_DIR' and the path used by 'install_deps.sh'.
This includes C-API bindings in intern/opensubdiv and CMAke module
which finds the OpenSubdiv library. This filea are not in use so
far, making it a separate commit to make actual integration commit
more clear.
