This is awesome. I didn't read the full proposal yet, but at first glance it's very close to how we do things where I work (because we needed something long before Node even existed, and never replaced it).

The biggest hurdle we constantly face is tools that make assumptions about the environment (eg: Flow, TypeScript, Webstorm, etc), so changing anything to module resolution breaks everything. We have to spend a lot of time hacking it in every single time. Sometimes the hooks just aren't there (TypeScript has a hook for overriding module resolution, but editors often don't give you an easy way to hook into TypeScript's own hooks).

Any thoughts on how things would work for tools if this was to go forward? Would they all need to be updated to honor this new way to resolve modules?