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...
i'd say we need a module(s) subdir, so we can do e.g.
```
from easybuild.tools.module.generator import ...
```
or even
```
from easybuild.module.generator import ...
```