uobject: Make module loader understand exports/imports.

uobject: Make module loader understand exports/imports.

It's possible for a package to have an instance of a class defined in
that same package; therefore the module loader needs to know about this.
Bring the export class index up to the public struct so that it's
possible to tell the difference.

This makes it possible to export the textures defined in Engine.u.
There's still a few vestigial bugs left over here, but we can fix those
later.