support for hierarchical module naming schemes

@geimer: this should fix the problem that's blocking you right now

I might need to figure out one more thing w.r.t. the devel module name before this really works for you, I'm looking into that...
It has to be absolute path imho, what other option do we have?

Doing these `prepend-path`s relative to `$root` would be worse imho? Since it would look something like (e.g. in the `GCC/4.8.2` module):

```tcl
prepend-path MODULEPATH $root/../../../modules/Compiler/GCC/4.8.2
```

vs (like it is now)

```
prepend-path MODULEPATH /tmp/prefix/modules/Compiler/GCC/4.8.2
```

If you're worried about not being able to relocate software, that's a can of worms anyway, since various software packages bake in the absolute install prefix anyway.