Generate a correct .flutter-plugin-dependencies file for iOS/macOS projects (#162834)

Closes https://github.com/flutter/flutter/issues/162704.

/cc @loic-sharma.

I expect I'll have to update some iOS/macOS unit and possibly
integration tests due to this change, but wanted something concrete to
talk about during our 1:1. Feel free to leave comments or questions even
if this PR is in "draft".

dev/devicelab/bin/tasks/build_ios_framework_module_test.dart

@@ -13,6 +13,11 @@ import 'package:path/path.dart' as path;
 /// Tests that iOS and macOS .xcframeworks can be built.
 Future<void> main() async {
   await task(() async {
+    // TODO(matanlurey): Remove after default.
+    // https://github.com/flutter/flutter/issues/160257
+    section('Opt-in to --explicit-package-dependencies');
+    await flutter('config', options: <String>['--explicit-package-dependencies']);
+
     section('Create module project');
 
     final Directory tempDir = Directory.systemTemp.createTempSync('flutter_module_test.');

dev/devicelab/bin/tasks/module_test_ios.dart

@@ -18,6 +18,11 @@ import 'package:path/path.dart' as path;
 /// adding Flutter to an existing iOS app.
 Future<void> main() async {
   await task(() async {
+    // TODO(matanlurey): Remove after default.
+    // https://github.com/flutter/flutter/issues/160257
+    section('Opt-in to --explicit-package-dependencies');
+    await flutter('config', options: <String>['--explicit-package-dependencies']);
+
     // Update pod repo.
     await eval(
       'pod',