* 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
master is only used in the context of `pkgs/override.nix`
and `unstableModulesPath`
and only within `./hosts`
Since overlays exists that are incompatible between nixos and master
at evaluation time (sic!), this together with #77 avoids evaluating
overlays against master all together.
since packages are exposed through output.packages by their own,
a second export through overlays.pkgs would be smudging the public api
of this repository.
