fix hierarchical mns w.r.t. MPI installed with a dummy toolchain

enhanced version of #983 by @kcgthb (which should have been opened targeting `develop`
@kcgthb: I updated this to throw an appropriate error whenever an MPI library being installed with a `dummy` module is being run into, since that's simply not compatible with this hierarchical module naming scheme...

I'm working on a set of easyconfig files for a new version of the `intel` toolchain that uses a proper `impi` easyconfig, i.e. one that uses an `iccifort` toolchain rather than `dummy`.