Move registry frontend code under `packages_and_registries`

With the addition of the infrastructure_registry and the dependency_proxy we have finalised a new root folder for the stage frontend code in packages_and_registries

The folder name mimics the menu entry in the product and makes it easier for any contributor to locate the relative code.

The container registry code sits outside of this folder in the registry folder.

We should move the registry folder to packages_and_registries/container_registry for consistency and easy to contribute.