Support for dummy toolchain in the hierarchical module naming system

Would that be a reasonable fix for #980 ? It seems to work but I have no idea about potential side effects.
We should probably short-circuit to returning `None` here, and then dealing with it where `det_toolchain_compilers_name_version` is being used (which is specific to `HierarchicalMNS`, it's not part of the module naming scheme API but internal to this Python module/class).