Reformat repository using new nixfmt-rfc-style.
Instead of having to manually stub packages that should not be downloaded we instead automatically stub all packages (except a small list of whitelisted ones). Tests can re-introduce the real package by using the `realPkgs` module argument.
- Add `skins` option for definition of multiple skin files - Change file extension to ".yaml" - Deprecate `skin` option (points to `skins.skin`)
