Add nix configuration for home-manager

This ensures home-manager devices use the the already downloaded nixpkgs
for ad-hoc commands like nix-shell and comma, as well as adds this flake
to the registry (currently assumed to be located at ~/nixfiles, I should
make this configurable later)

home/common/default.nix

@@ -3,5 +3,6 @@
   imports = [
     ./wm.nix
     ./nodm.nix
+    ./nix.nix
   ];
 }

home/common/nix.nix

@@ -0,0 +1,45 @@
+{ pkgs, lib, config, osConfig ? { }, options, nixpkgs, ... }:
+let
+  cfg = config.nixfiles.common.nix;
+  standalone = !(osConfig ? home-manager);
+in {
+  config = lib.mkIf cfg.enable (lib.mkMerge [
+    {
+      nix.registry = {
+        nixfiles = {
+          exact = true;
+          from = {
+            id = "nixfiles";
+            type = "indirect";
+          };
+          to = {
+            type = "git";
+            url = "file://${config.home.homeDirectory}/nixfiles";
+          };
+        };
+      };
+    }
+
+    (lib.mkIf standalone {
+      home.sessionVariables.NIX_PATH = "nixpkgs=${nixpkgs}\${NIX_PATH:+:\${NIX_PATH}}";
+      nix.registry = {
+        nixpkgs = {
+          exact = true;
+          from = {
+            id = "nixpkgs";
+            type = "indirect";
+          };
+          to = {
+            type = "github";
+            owner = "NixOS";
+            repo = "nixpkgs";
+            rev = "${nixpkgs.rev}";
+          };
+        };
+      };
+    })
+  ]);
+  options.nixfiles.common.nix = {
+    enable = lib.mkEnableOption "Nix configuration";
+  };
+}

home/profile/base.nix

@@ -15,6 +15,7 @@ in
 
   config = lib.mkIf cfg.enable {
     nixfiles.programs.comma.enable = true;
+    nixfiles.common.nix.enable = true;
 
     home.sessionVariables = lib.mkMerge [
       (lib.mkIf config.programs.neovim.enable {