Add skyx packages for all the Sky demos (except game)

sky/sdk/example/widgets/BUILD.gn

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-group("apps") {
+import("//sky/build/skyx.gni")
+
+skyx("sector") {
+  main_dart = "sector.dart"
+}
+
+skyx("piano") {
+  main_dart = "piano.dart"
+}
+
+group("widgets") {
   testonly = true
 
   deps = [
-    "//apps/moterm",
-    "//apps/moterm:apptests",
-    "//apps/moterm:gl_helper_test_app",
-    "//apps/moterm:moterm_example_app",
+    ":sector",
+    ":piano",
   ]
 }