* fix nixos-option
* use flake-compat for all backward compat concerns
* rebuild devshell after any nix file changes
* reexport system pkgs as legacyPackages
* provide lib flake output
* provide flake lib to nixos configs via `lib.flk`
* clean up flake.nix
In order to increase determinism in the build environment, we are
having direnv load the shell environment from the flake's `devShell`
output, instead of the traditional `nix-shell` approach. Of course,
one can still enter a `nix-shell` manually on a system without flake
support installed.
