Now that we have got two OpenCL ICD loaders in the tree, that starting with version 3, virtual/opencl will only pull an ICD loader rather than any specific implementation, and that we are in the process of following the footsteps of OpenGL in migrating away from using eselect to switch between OpenCL implementations in favour of always going through a loader, update implementations accordingly. Specifically: depend on >=virtual/opencl-3 rather than on any specific ICD loader, and make sure even implementations which previously ran standalone use one.
Note that while the same should be done for x11-drivers/nvidia-drivers, all the ebuilds there are marked stable so they will have to be handled with more care. Maintainers whose sign-off is needed: candrews for ROCm, zerochaos for intel-ocl-sdk, the X11 project for Mesa.
