Rename .sdkext to _sdkext and fix sky_tool

- Pub doesn't publish dot files (who knew?!)
- sky_tool was referencing a variable that doesn't exist

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1223173003 .

sdk/BUILD.gn

@@ -8,7 +8,7 @@ dart_pkg("sky") {
   sources = [
     "CHANGELOG.md",
     "bin/init.dart",
-    "lib/.sdkext",
+    "lib/_sdkext",
     "lib/animation/animated_value.dart",
     "lib/animation/animation_performance.dart",
     "lib/animation/curves.dart",

sdk/lib/sky_tool

@@ -135,7 +135,7 @@ class StartSky(object):
         if os.path.isdir(project_or_path):
             sky_server_root = project_or_path
             main_dart = os.path.join(project_or_path, 'lib', 'main.dart')
-            missing_msg = "Missing lib/main.dart in project: %s" % project_dir
+            missing_msg = "Missing lib/main.dart in project: %s" % project_or_path
         else:
             # FIXME: This assumes the path is at the root of the project!
             # Instead we should walk up looking for a pubspec.yaml