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`
@boegel: yeah, I was thinking maybe the module should end up in modules/all/Core, instead of a subdirectory of modules/all/MPI or modules/all/Compiler, since it hasn't be compiled with any of those.

But then for Intel MPI, for instance, it kinds of 'disconnects' it from the rest of the Intel toolchain, so I'm not really sure what's the best way to do this.