don't require Path::Class unless necessary

This patch changes `[Alien]` so that it only adds the dep on `Path::Class` if it is necessary.  I believe this is okay because the `gather_files` happens before `register_prereqs`.

I am a fan of `Path::Class`, but I was surprised that my Alien dist depended on it, even though it wasn't using it.  I can remove the dep in my `dist.ini`, but I don't think it should be there in the first place.

Thanks, I appreciate your consideration.