Requested by areinwrath to support something like a conkeror/site-js for local system scripts. Seems like
a good idea to me.

It is easy enough to make application.module_uri_prefix a list of prefixes, but now we need module name
conflict resolution. It would probably be useful to use emacs' behavior as a starting point.

The current practice of tracking modules purely by module name (rather than by full module path) will
probably need to change.