Automatically wire dependencies for native plugins (#8891)

Go through all packages brought in by pub, and write the name and path of every one that is a flutter plugin into .flutter-plugins.

In android/settings.gradle and ios/Podfile, read in .flutter-plugins, if that file exists. The Android / iOS code from the plugins is automatically added as dependencies of the native code of the app.

packages/flutter_tools/lib/src/ios/mac.dart

diff --git a/packages/flutter_tools/lib/src/ios/mac.dart b/packages/flutter_tools/lib/src/ios/mac.dart
index e20e793..e26e0d7 100644
--- a/packages/flutter_tools/lib/src/ios/mac.dart
+++ b/packages/flutter_tools/lib/src/ios/mac.dart
@@ -19,6 +19,7 @@
 import '../doctor.dart';
 import '../flx.dart' as flx;
 import '../globals.dart';
+import '../plugins.dart';
 import '../services.dart';
 import 'xcodeproj.dart';
 
@@ -127,6 +128,7 @@
   // copied over to a location that is suitable for Xcodebuild to find them.
   final Directory appDirectory = fs.directory(app.appDirectory);
   await _addServicesToBundle(appDirectory);
+  writeFlutterPluginsList();
 
   _runPodInstall(appDirectory, flutterFrameworkDir(mode));