Rename extensions to include MPI in name

Developing a package that also requires AMDGPU and CUDA extensions and MPI, we hit an issue about conflicting extension names that can't be resolved.

A solution is to name the extensions in a way to include the package name they are belonging to. For MPI, it means renaming extensions to `MPIAMDGPUext` and `MPICUDAext`.

If this sounds good I could draft a PR.