Require module_id param to resolve to be non-empty

Previously, `resolve` would immediately check that `module_id` was
non-empty and give an error if not. This had two downsides:

- It introduced `Option`s everywhere, even if the calling function knew
it had a valid module, and
- It checked the module on each namespace, which is unnecessary: it only
needed to be checked once.

This makes the caller responsible for checking the module exists, making
the code a lot simpler.