getLazySingleton<T extends Object>  method 
Retrieves the lazily loaded singleton dependency.
Implementation
Option<Resolvable<T>> getLazySingleton<T extends Object>({
  Entity groupEntity = const DefaultEntity(),
  bool traverse = true,
}) {
  final option = getLazy<T>(groupEntity: groupEntity, traverse: traverse);
  if (option.isNone()) {
    return const None();
  }
  UNSAFE:
  final lazy = option.unwrap().sync().unwrap().unwrap();
  return Some(lazy.singleton);
}