static boolean suitableForBuilderConstructor(ExecutableElement element) {
      return element.getKind() == ElementKind.CONSTRUCTOR
          && !element.getModifiers().contains(Modifier.PRIVATE);
    }
Checks if this element is a regular POJO (not an interface or abstract class) simple
class with getters and setters
value-processor/src/org/immutables/value/processor/meta/Proto.java