code: public ImmutableSet<ClassInfo> getTopLevelClassesRecursive(String packageName) {
      checkNotNull(packageName);
      String packagePrefix = packageName + '.';
      ImmutableSet.Builder<ClassInfo> builder = ImmutableSet.builder();
      for (ClassInfo classInfo : getTopLevelClasses()) {
        if (classInfo.getName().startsWith(packagePrefix)) {
          builder.add(classInfo);
        }
      }
      return builder.build();
    }
docs: Returns all top level classes whose package name is {@code packageName} or starts with {@code
  packageName} followed by a '.'.