Support for root packages for core24

[sergiusens]

What needs to get done

In the non Craft Application based snapcraft root level build-packages and stage-packages are bundled into a part before being passed to Craft Parts. This logic is missing in the new Craft Application based Snapcraft

Why it needs to get done

Implement the similar logic to https://github.com/canonical/snapcraft/blob/8a9a5c21229f68a5e0daa7c89d609d7d29006477/snapcraft/parts/yaml_utils.py#L186 and its similar unit tests which would allow tests/spread/core24/root-packages/task.yaml to pass