feat!(flake): convert to fs-defined configurations

Place all configurations in `machines/${manager}/${system}/${hostname}/...`:
    - `${manager}` - One of `nixos`, `nix-on-droid`, `nix-darwin` or `home-manager`
    - `${system}` - A system's architecture (can be many)
    - `${hostname}` - A system's hostname (can be many)

The flake now automatically generates the needed configurations based on the above structure.

It only generates configurations for valid directory structures:
    - It wouldn't generate a NixOS config if there isn't a `configuration.nix` file
    - ...

templates/nixos/arthur/configuration.nix

@@ -0,0 +1,99 @@
+# This is your system's configuration file.
+# Use this to configure your system environment (it replaces /etc/nixos/configuration.nix)
+
+{ inputs, outputs, lib, config, pkgs, ... }: {
+  # You can import other NixOS modules here
+  imports = [
+    # If you want to use modules your own flake exports (from modules/nixos):
+    # outputs.nixosModules.example
+
+    # Or modules from other flakes (such as nixos-hardware):
+    # inputs.hardware.nixosModules.common-cpu-amd
+    # inputs.hardware.nixosModules.common-ssd
+
+    # You can also split up your configuration and import pieces of it here:
+    # ./users.nix
+
+    # Import your generated (nixos-generate-config) hardware configuration
+    ./hardware-configuration.nix
+  ];
+
+  nixpkgs = {
+    # You can add overlays here
+    overlays = [
+      # If you want to use overlays your own flake exports (from overlays dir):
+      # outputs.overlays.modifications
+      # outputs.overlays.additions
+
+      # Or overlays exported from other flakes:
+      # neovim-nightly-overlay.overlays.default
+
+      # Or define it inline, for example:
+      # (final: prev: {
+      #   hi = final.hello.overrideAttrs (oldAttrs: {
+      #     patches = [ ./change-hello-to-hi.patch ];
+      #   });
+      # })
+    ];
+    # Configure your nixpkgs instance
+    config = {
+      # Disable if you don't want unfree packages
+      allowUnfree = true;
+    };
+  };
+
+  nix = {
+    # This will add each flake input as a registry
+    # To make nix3 commands consistent with your flake
+    registry = lib.mapAttrs (_: value: { flake = value; }) inputs;
+
+    # This will additionally add your inputs to the system's legacy channels
+    # Making legacy nix commands consistent as well, awesome!
+    nixPath = lib.mapAttrsToList (key: value: "${key}=${value.to.path}") config.nix.registry;
+
+    settings = {
+      # Enable flakes and new 'nix' command
+      experimental-features = "nix-command flakes";
+      # Deduplicate and optimize nix store
+      auto-optimise-store = true;
+    };
+  };
+
+  # FIXME: Add the rest of your current configuration
+
+  # TODO: Set your hostname
+  networking.hostName = "your-hostname";
+
+  # TODO: This is just an example, be sure to use whatever bootloader you prefer
+  boot.loader.systemd-boot.enable = true;
+
+  # TODO: Configure your system-wide user settings (groups, etc), add more users as needed.
+  users.users = {
+    # FIXME: Replace with your username
+    your-username = {
+      # TODO: You can set an initial password for your user.
+      # If you do, you can skip setting a root password by passing '--no-root-passwd' to nixos-install.
+      # Be sure to change it (using passwd) after rebooting!
+      # initialPassword = "correcthorsebatterystaple";
+      isNormalUser = true;
+      openssh.authorizedKeys.keys = [
+        # TODO: Add your SSH public key(s) here, if you plan on using SSH to connect
+      ];
+      # TODO: Be sure to add any other groups you need (such as networkmanager, audio, docker, etc)
+      extraGroups = [ "wheel" ];
+    };
+  };
+
+  # This setups a SSH server. Very important if you're setting up a headless system.
+  # Feel free to remove if you don't need it.
+  services.openssh = {
+    enable = true;
+    # Forbid root login through SSH.
+    permitRootLogin = "no";
+    # Use keys only. Remove if you want to SSH using password (not recommended)
+    passwordAuthentication = false;
+  };
+
+  # https://nixos.wiki/wiki/FAQ/When_do_I_update_stateVersion
+  system.stateVersion = "22.05";
+}

templates/nixos/arthur/hardware-configuration.nix

@@ -0,0 +1,10 @@
+# This is just an example, you should generate yours with nixos-generate-config and put it in here.
+{
+  fileSystems."/" = {
+    device = "/dev/sda1";
+    fsType = "ext4";
+  };
+
+  # Set your system kind (needed for flakes)
+  nixpkgs.hostPlatform = "x86_64-linux";
+}