private static ImmutableSortedSet.Builder<TypeMirror> newTypeSetBuilder() {
    return ImmutableSortedSet.orderedBy(
        Comparator.comparing(t -> MoreTypes.asTypeElement(t).getQualifiedName().toString()));
  }
If non-null, we will call this whenever the {@link #process} method is called, giving it one of
the {@code @AutoFactory} elements, and do nothing else.
factory/src/main/java/com/google/auto/factory/processor/AutoFactoryProcessor.java