cray-mpich: fix dependencies for externals from manifest (#34231)

The cray manifest shows dependency information for cray-mpich, which we never previously cared about
because it can only be used as an external. This updates Spack's dependency information to make cray-mpich
specs read in from the cray external manifest usable.

var/spack/repos/builtin/packages/cray-mpich/package.py

@@ -29,6 +29,9 @@ class CrayMpich(Package):
     version("7.7.14")
     version("7.7.13")
 
+    depends_on("cray-pmi")
+    depends_on("libfabric")
+
     # cray-mpich 8.1.7: features MPI compiler wrappers
     variant("wrappers", default=True, when="@8.1.7:", description="enable MPI wrappers")