Allow add-to-app modules to use plugins that don't support iOS (.flutter-plugins-dependencies)

https://github.com/flutter/flutter/issues/39659 but for add-to-app.  Do something like https://github.com/flutter/flutter/pull/59209 to parse `.flutter-plugins-dependencies` in the module podhelper.