ModuleDescriptor retains overlapping sets for all and concealed packages

ModuleDescriptor eagerly calculate and retain sets for packages() and conceals(), although these often overlap. A partial optimization was delivered to de-duplicate these sets when a module didn't export anything, but that only helped cut down footprint of a few modules.

While only maintaining a set of the concealed packages would be optimal from a footprint perspective, maintaining the set of all packages and only calculating the set of concealed packages on demand has benefits to startup time.