Background

There are currently two ways to embed plugins in an iOS app via CocoaPods.

1. flutter created apps import a helper Ruby script into their ios/Podfile that lives in the flutter_tools package bin directory, and then helper methods like flutter_install_all_ios_pods are called.

2. The second way is how add-to-app host apps embed plugins. Host apps also import a helper Ruby script into their Podfile and call helper methods like install_all_flutter_pods, but this helper script is generated in the Flutter module itself as a template file. This allows the host app to import the script as a relative path, and avoids absolute Flutter SDK paths from being checked anywhere into the host app project.
   

   flutter/dev/integration_tests/ios_host_app/Podfile

   Lines 4 to 8 in 80f15dc

   	load File.join(flutter_application_path, '.ios', 'Flutter', 'podhelper.rb')
   	target 'Host' do
   	install_all_flutter_pods flutter_application_path
   	end

The Flutter module (flutter create -t module) app generates an ephemeral fake-host app (recreated on every flutter clean) to allow fast flutter run prototyping of the Flutter bits. This "host" app currently uses the second plugin embedding method and uses the generated template podhelper.rb. This PR changes it to use the first method to more closely align it to a normal flutter create'd app. This will allow the module to leverage the new Flutter XCFramework logic, and avoid issues like #37850 which have been resolved in the first method.

Description

In the add-to-app module Podfile use the Flutter SDK podhelper helper methods to match the behavior of normal apps, instead of using the podhelper that's generated in the module, which will still be imported in the host app's Podfile.

1. Update module plugin symlinks directory to match apps, .ios/Flutter/.symlinks/ -> .ios/.symlinks/plugins/
2. Remove extraneous engine copy from build ios-frameworks since the plugins now link on the engine artifacts instead of the one copied into the module app.
3. Change the module Podfile to pretty closely mimic a regular app Podfile, with the addition of the FlutterPluginRegistrant pod.
4. Default to copying release framework that contains bitcode, instead of copying the debug framework.

Related Issues

Fixes flutter build ios --release --local-engine in modules since the plugins are linking directly against the engine artifacts. #51989
Fixes #48092

Tests

Added checks for the host app Podfile.lock in module_test_ios.
Thoroughly tested by build_ios_framework_module_test.