Index-based Downloads Client Implementations
define format (config/plugin) that can be used to inject additional downloads: TODO
define way to limit it only to certain hosts/origins: TODO
add code in apt that actually downloads the additional files: TODO
ensure that the list-cleanup behavior is sane and no removal happens: TODO
ensure that the correct "index-file: yes" is passed to the backend method: TODO
* Blueprints in grey have been implemented.