Make module pod headers public (#40927)

packages/flutter_tools/templates/module/ios/host_app_ephemeral_cocoapods/Podfile.copy.tmpl

@@ -3,10 +3,24 @@ platform :ios, '8.0'
 flutter_application_path = '../'
 load File.join(flutter_application_path, '.ios', 'Flutter', 'podhelper.rb')
 
+use_frameworks!
+
 target 'Runner' do
   install_flutter_engine_pod
   install_flutter_plugin_pods flutter_application_path
 end
 
+post_install do |installer|
+  installer.pods_project.targets.each do |target|
+    # Aggregate targets do not have a headers build phase.
+    if target.respond_to?('headers_build_phase')
+      target.headers_build_phase.files.each do |file|
+        # Make headers public so they can be imported by the host application.
+        file.settings = { 'ATTRIBUTES' => ['Public'] }
+      end
+    end
+  end
+end
+
 # Prevent Cocoapods from embedding a second Flutter framework and causing an error with the new Xcode build system.
 install! 'cocoapods', :disable_input_output_paths => true