Added a getPackageNames function to the PackageSupplier interface, and an implementation for the RegistryPackageSupplier. Currently, FileSystemPackageSupplier.getPackageNames is not implemented.

The catch block of Dub.createEmptyPackage() now uses this and tries to find close matches to what the user entered.
Currently, "close matches" are defined as anything with a levenshteinDistance of <=4.