I think it'll be okay for now to not support full resolution for addons, but we should still use that method from `toolkit/loader` to smooth out the differences between `./index.js` and `index.js` (the latter is more common), and does not require any IO. My only concern with this is consuming an existing node module that has a directory as a main.

Maybe `getManifestMain` can handle the dir use case (will need added) for dependencies, but loading an addon in AOM will require a file and not need IO, so the constraint is only for addons, not for SDK specific utilities or JS modules on npm