Extract missing module support in hierarchy.cc to a helper function

Extract missing module support in hierarchy.cc to a helper function

I think the code is now a bit easier to follow (and has lost some
levels of indentation!).

The only non-trivial change is that I removed the check for
cell->type[0] != '$' when deciding whether to complain if we couldn't
find a module. This will always be true because of the early exit
earlier in the function.