Move all packages under packages folder to follow popular Flutter project structure convention

[albertus-stripe]

I have a very nitpick suggestion on improving our current project folder structure. Since this project publishes multiple packages at the same time, what do you think about adopting the flutter/flutter folder structure and moving stripe, stripe_android, stripe_ios, and stripe_platform_interfaces inside the packages folder?

Some references on Flutter project repository following the same convention:

1. Flutter SDK
2. Flutter Plugins
3. FlutterFire
4. plus_plugins

(discussed with the maintainer offline and we agreed on doing this later to prevent conflicts on current branch)