Allow the build directory of rmm to be used for find_package(rmm)

[robertmaynard]

This folds into the rework for libcudf to support export targets ( rapidsai/cudf#7107 ).

Developers that are iterating on rmm and cudf should be able cudf against a local build of rmm without having to re-install rmm each time they change a rmm header. This MR supports this use-case by allowing find_package(rmm) to work with a rmm build directory.

To verify this ( using rapidsai/cudf#7107 ) the following will make sure cudf detects your local rmm instead of fetching it via CPM.

cmake -S cudf/cpp -B cudf/cpp/cudf -DCMAKE_PREFIX_PATH=<rmm_build_dir>

[germasch]

FWIW, looks good to me ;)

[trxcllnt]

🙏

[kkraus14]

@gpucibot merge