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

sky/sdk/example/rendering/BUILD.gn

Index: sky/sdk/example/rendering/BUILD.gn
diff --git a/apps/BUILD.gn b/sky/sdk/example/rendering/BUILD.gn
similarity index 57%
copy from apps/BUILD.gn
copy to sky/sdk/example/rendering/BUILD.gn
index 59db4eefb1910915c21789af1cd6f423cf602237..3cadbad52ad827f584d4aca94d881db5e74490fb 100644
--- a/apps/BUILD.gn
+++ b/sky/sdk/example/rendering/BUILD.gn
@@ -2,13 +2,16 @@
 # 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("interactive_flex") {
+  main_dart = "interactive_flex.dart"
+}
+
+group("rendering") {
   testonly = true
 
   deps = [
-    "//apps/moterm",
-    "//apps/moterm:apptests",
-    "//apps/moterm:gl_helper_test_app",
-    "//apps/moterm:moterm_example_app",
+    ":interactive_flex",
   ]
 }