Re-organize flutter_tools code to better support proper layering. (#12957)

* executable.dart#main() depends on runner.dart#run()
* Refactor code such that non-commands don't depend on commands.

No code was actually changed in this PR - code was merely moved from
point A to point B.

packages/flutter_tools/lib/src/run_cold.dart

diff --git a/packages/flutter_tools/lib/src/run_cold.dart b/packages/flutter_tools/lib/src/run_cold.dart
index 76d8bd6..713d61c 100644
--- a/packages/flutter_tools/lib/src/run_cold.dart
+++ b/packages/flutter_tools/lib/src/run_cold.dart
@@ -7,10 +7,10 @@
 import 'package:meta/meta.dart';
 
 import 'base/file_system.dart';
-import 'commands/trace.dart';
 import 'device.dart';
 import 'globals.dart';
 import 'resident_runner.dart';
+import 'tracing.dart';
 
 class ColdRunner extends ResidentRunner {
   ColdRunner(