8034245: Refactor TopLevel tree node

Added JCPackageDecl to JCTree to store package-level information.

Reviewed-by: jjg, jlahoda

langtools/src/share/classes/com/sun/source/tree/Tree.java

@@ -207,6 +207,12 @@ public interface Tree {
          */
         LAMBDA_EXPRESSION(LambdaExpressionTree.class),
 
+        /**
+         * Used for instances of {@link PackageTree}.
+         * @since 1.9
+         */
+        PACKAGE(PackageTree.class),
+
         /**
          * Used for instances of {@link ParenthesizedTree}.
          */