Introduced named repositories (avoids repeated entry of urls);
simplified DirectorArguments and InstallArguments by removing factory
methods (clients use public constructors instead); properties of
@Immutable classes are now read-only (using final) to prevent exposing
setter methods; DirectorArguments and InstallArguments both delegate to
new RepositoryDelegate class to remove duplicate code
(RepositoryDelegate uses new Repository class); RepositoryDelegate and
Repository use properties instead of fields where appropriate, in
combination with @TupleConstructor; renamed InstallableUnitArguments to
InstallableUnit