Per-package name resolution

[rtfeldman]

Currently, if I have two modules both named Event, one in both my app and in the other in the platform it uses, I'll have a naming collision.

This should not be true! Both Event modules should be able to coexist (even if the platform exposes that Event module), because the platform doesn't know about the app's Event, and the app needs to qualify platform modules with a shorthand (e.g. pf.Event for the platform's version, and Event for the app's own version).

Another symptom of this is that currently when I'm writing my platform, I have to write (for example) pf.Task inside the platform modules themselves, in order to have them work when building the app. However, this means when running (for example) roc check on the platform by itself, it fails because the shorthand pf is not defined!