static ImmutableSet<ExecutableElement> prefixedGettersIn(Collection<ExecutableElement> methods) {
    return methods.stream()
        .filter(AutoValueishProcessor::isPrefixedGetter)
        .collect(toImmutableSet());
  }
Returns the subset of the given zero-arg methods whose names begin with {@code get}. Also
includes {@code isFoo} methods if they return {@code boolean}. This corresponds to JavaBeans
conventions.
value/src/main/java/com/google/auto/value/processor/AutoValueishProcessor.java