nixos: create 'modules/nixos' folder

Let's consolidate all modules under one path, so that NixOS,
home-manager, and nix-darwin (if I ever end up using it down the line)
would go under the same folder.

modules/nixos/system/nix/default.nix

@@ -0,0 +1,107 @@
+# Nix related settings
+{ config, inputs, lib, options, pkgs, ... }:
+let
+  cfg = config.my.system.nix;
+
+  channels = lib.my.merge [
+    {
+      # Allow me to use my custom package using `nix run self#pkg`
+      self = inputs.self;
+      # Add NUR to run some packages that are only present there
+      nur = inputs.nur;
+      # Use pinned nixpkgs when using `nix run pkgs#<whatever>`
+      pkgs = inputs.nixpkgs;
+    }
+    (lib.optionalAttrs cfg.inputs.overrideNixpkgs {
+      # ... And with `nix run nixpkgs#<whatever>`
+      nixpkgs = inputs.nixpkgs;
+    })
+  ];
+in
+{
+  options.my.system.nix = with lib; {
+    enable = my.mkDisableOption "nix configuration";
+
+    cache = {
+      selfHosted = my.mkDisableOption "self-hosted cache";
+    };
+
+    inputs = {
+      link = my.mkDisableOption "link inputs to `/etc/nix/inputs/`";
+
+      addToRegistry = my.mkDisableOption "add inputs and self to registry";
+
+      addToNixPath = my.mkDisableOption "add inputs and self to nix path";
+
+      overrideNixpkgs = my.mkDisableOption "point nixpkgs to pinned system version";
+    };
+  };
+
+  config = lib.mkIf cfg.enable (lib.mkMerge [
+    {
+      assertions = [
+        {
+          assertion = cfg.inputs.addToNixPath -> cfg.inputs.link;
+          message = ''
+            enabling `my.system.nix.inputs.addToNixPath` needs to have
+            `my.system.nix.inputs.link = true`
+          '';
+        }
+      ];
+    }
+
+    {
+      nix = {
+        package = pkgs.nix;
+
+        settings = {
+          experimental-features = [ "nix-command" "flakes" ];
+        };
+      };
+    }
+
+    (lib.mkIf cfg.cache.selfHosted {
+      nix = {
+        settings = {
+          # The NixOS module adds the official Hydra cache by default
+          # No need to use `extra-*` options.
+          substituters = [
+            "https://cache.belanyi.fr/"
+          ];
+
+          trusted-public-keys = [
+            "cache.belanyi.fr:LPhrTqufwfxTceg1nRWueDWf7/2zSVY9K00pq2UI7tw="
+          ];
+        };
+      };
+    })
+
+    (lib.mkIf cfg.inputs.addToRegistry {
+      nix.registry =
+        let
+          makeEntry = v: { flake = v; };
+          makeEntries = lib.mapAttrs (lib.const makeEntry);
+        in
+        makeEntries channels;
+    })
+
+    (lib.mkIf cfg.inputs.link {
+      environment.etc =
+        let
+          makeLink = n: v: {
+            name = "nix/inputs/${n}";
+            value = { source = v.outPath; };
+          };
+          makeLinks = lib.mapAttrs' makeLink;
+        in
+        makeLinks channels;
+    })
+
+    (lib.mkIf cfg.inputs.addToNixPath {
+      nix.nixPath = [
+        "/etc/nix/inputs"
+      ]
+      ++ options.nix.nixPath.default;
+    })
+  ]);
+}