Make ExtendedNode and Label public again (#6130)

In #5955, the `ExtendedNode` and `Label` classes from the dataflow
framework were changed from public to package-private. However, NullAway
relies on these classes being public (see
#5156,
#5152, and
uber/NullAway#608). This PR makes the classes
public again and adds an explanatory comment.

dataflow/src/main/java/org/checkerframework/dataflow/cfg/builder/ExtendedNode.java

@@ -18,9 +18,12 @@
  *   <li><em>TWO_TARGET_CONDITIONAL_JUMP</em>: {@link ConditionalJump}. A conditional jump with two
  *       targets for both the 'then' and 'else' branch.
  * </ul>
+ *
+ * Note that this class is deliberately public, to enable users of the dataflow library to customize
+ * CFG construction.
  */
 @SuppressWarnings("nullness") // TODO
-/*package-private*/ abstract class ExtendedNode {
+abstract class ExtendedNode {
 
   /** The basic block this extended node belongs to (as determined in phase two). */
   protected BlockImpl block;

dataflow/src/main/java/org/checkerframework/dataflow/cfg/builder/Label.java

@@ -4,8 +4,11 @@
  * A label is used to refer to other extended nodes using a mapping from labels to extended nodes.
  * Labels get their names either from labeled statements in the source code or from internally
  * generated unique names.
+ *
+ * <p>Note that this class is deliberately public, to enable users of the dataflow library to
+ * customize CFG construction.
  */
-/*package-private*/ class Label {
+class Label {
 
   /** Unique id counter that incremented in {@code #uniqueName}. */
   private static int uid = 0;