When moduledir is relative, prepend the basedir

The basic premise works, however right now we're appending the moduledir
again at the end. That shouldn't happen (imo), but maybe I'm
misunderstanding some core-concept of r10k here.

In general, this fixes #35 and when used with `moduledir ''` it also accommodates for different use-cases, such as managing more than just modules with r10k.