Flake outputs are a mixture of system-dependent and system-independent
sets, and flake-utils doesn't do much to distinguish one from the other.
Because of that, the `age` NixOS module currently has to be acessed as
`agenix.nixosModules.${system}.age`, rather than the documented
`agenix.nixosModules.age`.
To remedy that, (conceptually) split `outputs` in two, let flake-utils
handle the system-dependent half, and merge them to form the actual
outputs. The names for the two halves were taken from [1].
[1]: https://github.com/NixOS/nix/issues/3843#issuecomment-661720562