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.
Before it was not possible to place setting values outside the `General` section.
Add missing `tag` option for elements of `programs.pet.snippets`.
