Relative module paths

Instead of internally referring to a global `nbd` path, we make it more generic by using relative path dependencies.

This changes the paths minimally, except for using `build/all` to specify all modules. Since the module is used in only two places, the build script and the test launcher, we can enforce a `nbd` path in both places. We still want the `build/all` module to point to `nbd` paths so as to be able to redirect it for now. Perhaps in the future we won't need to.