Update lang/nix to reflect changes upstream

+ Restore nix-mode entry in auto-mode-alist
+ Add .drv entry to auto-mode-alist
+ Minor refactor for def-package! blocks

modules/lang/nix/config.el

@@ -1,8 +1,10 @@
 ;;; lang/nix/config.el -*- lexical-binding: t; -*-
 
-(after! nix-mode
+(def-package! nix-mode
+  :mode "\\.nix\\'"
+  :config
   (set-company-backend! 'nix-mode 'company-nixos-options)
-        
+
   (map! :map nix-mode-map
         :localleader
         :n "f" #'nix-update-fetch
@@ -14,8 +16,11 @@
         (:when (featurep! :completion helm)
           :n "o" #'helm-nixos-options)))
 
+(def-package! nix-drv-mode
+  :mode "\\.drv\\'")
+
 (def-package! nix-update
-  :commands (nix-update-fetch))
+  :commands nix-update-fetch)
 
 (def-package! nix-repl
-  :commands (nix-repl-show))
+  :commands nix-repl-show)