private static ImmutableMap<String, TypeElement> apiMap(
      Elements elementUtils, String apiPackage) {
    return API_CLASSES.stream()
        .map(name -> new SimpleEntry<>(name, elementUtils.getTypeElement(apiPackage + name)))
        .filter(entry -> entry.getValue() != null)
        .collect(toImmutableMap(SimpleEntry::getKey, SimpleEntry::getValue));
  }
True if {@code type} is a {@code Provider}.
factory/src/main/java/com/google/auto/factory/processor/InjectApi.java