Define repository content through CR

[yselkowitz]

Currently, package lists need to be managed separately in CR, pungi configs and overrides, and comps, which is a bit ridiculous and definitely time-consuming and error-prone. We would like to have CR be the single source of truth which defines the contents of ELN or c10s. This will certainly require (perhaps extensive) changes to CR-input as well as the code, so this needs to be designed well in advance.