Rename scroll.dart to drag.dart

The gesture is now called "drag". The library name should match.

packages/flutter/lib/src/widgets/gesture_detector.dart

@@ -4,9 +4,9 @@
 
 import 'dart:sky' as sky;
 
+import 'package:sky/gestures/drag.dart';
 import 'package:sky/gestures/long_press.dart';
 import 'package:sky/gestures/recognizer.dart';
-import 'package:sky/gestures/scroll.dart';
 import 'package:sky/gestures/show_press.dart';
 import 'package:sky/gestures/tap.dart';
 import 'package:sky/src/rendering/sky_binding.dart';

packages/flutter/lib/src/widgets/scrollable.dart

@@ -9,7 +9,7 @@ import 'dart:sky' as sky;
 import 'package:newton/newton.dart';
 import 'package:sky/animation.dart';
 import 'package:sky/gestures/constants.dart';
-import 'package:sky/gestures/scroll.dart';
+import 'package:sky/gestures/drag.dart';
 import 'package:sky/src/rendering/box.dart';
 import 'package:sky/src/rendering/viewport.dart';
 import 'package:sky/src/widgets/basic.dart';

packages/unit/test/gestures/scroll_test.dart

 import 'dart:sky' as sky;
 
 import 'package:sky/gestures/arena.dart';
+import 'package:sky/gestures/drag.dart';
 import 'package:sky/gestures/pointer_router.dart';
-import 'package:sky/gestures/scroll.dart';
 import 'package:sky/gestures/tap.dart';
 import 'package:test/test.dart';