Merge shared module back into the NixOS module

Since nix-darwin no longer needs these.

modules/nixos.nix

@@ -1,12 +1,64 @@
 inputs:
 { lib, pkgs, config, ... }:
 let
-  inherit (import ./shared.nix inputs)
-    commonNixSettingsModule
-    restrictedNixSettingsModule
-    mkPreferable
-    mkMorePreferable
-  ;
+  # Stronger than mkDefault (1000), weaker than mkForce (50) and the "default override priority"
+  # (100).
+  mkPreferable = lib.mkOverride 750;
+
+  # Stronger than the "default override priority", as the upstream module uses that, and weaker than mkForce (50).
+  mkMorePreferable = lib.mkOverride 75;
+
+  # The settings configured in this module must be generally settable by users both trusted and
+  # untrusted by the Nix daemon. Settings that require being a trusted user belong in the
+  # `restrictedSettingsModule` below.
+  commonNixSettingsModule = { config, pkgs, lib, ... }: {
+    nix.package = inputs.nix.packages."${pkgs.stdenv.system}".default;
+
+    nix.registry.nixpkgs = {
+      exact = true;
+
+      from = {
+        type = "indirect";
+        id = "nixpkgs";
+      };
+
+      # NOTE(cole-h): The NixOS module exposes a `flake` option that is a fancy wrapper around
+      # setting `to` -- we don't want to clobber this if users have set it on their own
+      to = lib.mkIf (config.nix.registry.nixpkgs.flake or null == null) (mkPreferable {
+        type = "tarball";
+        url = "https://flakehub.com/f/DeterminateSystems/nixpkgs-weekly/0.1.0.tar.gz";
+      });
+    };
+
+    nix.settings = {
+      bash-prompt-prefix = "(nix:$name)\\040";
+      extra-experimental-features = [ "nix-command" "flakes" ];
+      extra-nix-path = [ "nixpkgs=flake:nixpkgs" ];
+      extra-substituters = [ "https://cache.flakehub.com" ];
+    };
+  };
+
+  # The settings configured in this module require being a user trusted by the Nix daemon.
+  restrictedNixSettingsModule = { ... }: {
+    nix.settings = restrictedNixSettings;
+  };
+
+  # Nix settings that require being a trusted user to configure.
+  restrictedNixSettings = {
+    always-allow-substitutes = true;
+    netrc-file = "/nix/var/determinate/netrc";
+    upgrade-nix-store-path-url = "https://install.determinate.systems/nix-upgrade/stable/universal";
+    extra-trusted-public-keys = [
+      "cache.flakehub.com-3:hJuILl5sVK4iKm86JzgdXW12Y2Hwd5G07qKtHTOcDCM="
+      "cache.flakehub.com-4:Asi8qIv291s0aYLyH6IOnr5Kf6+OF14WVjkE6t3xMio="
+      "cache.flakehub.com-5:zB96CRlL7tiPtzA9/WKyPkp3A2vqxqgdgyTVNGShPDU="
+      "cache.flakehub.com-6:W4EGFwAGgBj3he7c5fNh9NkOXw0PUVaxygCVKeuvaqU="
+      "cache.flakehub.com-7:mvxJ2DZVHn/kRxlIaxYNMuDG1OvMckZu32um1TadOR8="
+      "cache.flakehub.com-8:moO+OVS0mnTjBTcOUh2kYLQEd59ExzyoW1QgQ8XAARQ="
+      "cache.flakehub.com-9:wChaSeTI6TeCuV/Sg2513ZIM9i0qJaYsF+lZCXg0J6o="
+      "cache.flakehub.com-10:2GqeNlIp6AKp4EF2MVbE1kBOp9iBSyo0UPR9KoR0o1Y="
+    ];
+  };
 in
 {
   imports = [