protected Iterable<? extends Step> steps() {
    return Iterables.transform(initSteps(), BasicAnnotationProcessor::asStep);
  }
Creates {@linkplain Step processing steps} for this processor. {@link #processingEnv} is
guaranteed to be set when this method is invoked.

<p>Note: If you are migrating some steps from {@link ProcessingStep} to {@link Step}, then you
can call {@link #asStep(ProcessingStep)} on any unmigrated steps.
common/src/main/java/com/google/auto/common/BasicAnnotationProcessor.java