* 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
Create fetch function which takes a valid flake input, and calls
fetchTarball with the revision and hash information from the flake.lock.
Useful for pulling the exact revision of flake inputs from not flake
expressions, such as those in shell.nix.