Charm++: fix build prefix; add ucx/pmi support (#15666)

* Add pmi support (required by ucx, ofi, and gni backends)
* Add support for ucx backend
* Add dependency on MPI for pmi=simplepmi, slurmpmi, or slurmpmi2
* Remove charmpp as an MPI provider since the changes in this PR can
  add MPI as a dependency (mentioned previously)
* Install into transport_protocol-OS-arch subdirectory to match
  default charmpp installation behavior (which helps dependents find it)

lib/spack/spack/test/cmd/providers.py

@@ -22,8 +22,7 @@ def test_it_just_runs(pkg):
 
 
 @pytest.mark.parametrize('vpkg,provider_list', [
-    (('mpi',), ['charmpp@6.7.1:',
-                'intel-mpi',
+    (('mpi',), ['intel-mpi',
                 'intel-parallel-studio',
                 'mpich',
                 'mpich@1:',