Declaratively manage plugins not in nixpkgs

Created the plugins/plugin-defs.nix file, which will include
definitions for all plugins not in nixpkgs. This way, we can get
rid of the packer dependency and make the whole thing truly
declarative!

plugins/plugin-defs.nix

@@ -0,0 +1,16 @@
+# This is for plugins not in nixpkgs
+# e.g. intellitab.nvim
+# Ideally, in the future, this would all be specified as a flake input!
+{ pkgs, ... }:
+{
+  intellitab-nvim = pkgs.vimUtils.buildVimPlugin rec {
+    pname = "intellitab-nvim";
+    version = "a6c1a505865f6131866d609c52440306e9914b16";
+    src = pkgs.fetchFromGitHub {
+      owner = "pta2002";
+      repo = "intellitab.nvim";
+      rev = version;
+      sha256 = "19my464jsji7cb81h0agflzb0vmmb3f5kapv0wwhpdddcfzvp4fg";
+    };
+  };
+}

plugins/utils/intellitab.nix

@@ -2,7 +2,7 @@
 with lib;
 let
   cfg = config.programs.nixvim.plugins.comment-nvim;
-  helpers = import ../helpers.nix { inherit lib; };
+  defs = import ../plugin-defs.nix { inherit pkgs; };
 in
 {
   options = {
@@ -13,12 +13,9 @@ in
 
   config = mkIf cfg.enable {
     programs.nixvim = {
-      maps.insert."<Tab>" = "<CMD>lua require([[intellitab]]).indent()<CR>";
-      plugins.packer = {
-        enable = true;
-        plugins = [ "pta2002/intellitab.nvim" ];
-      };
+      extraPlugins = [ defs.intellitab-nvim ];
 
+      maps.insert."<Tab>" = "<CMD>lua require([[intellitab]]).indent()<CR>";
       plugins.treesitter = {
         indent = true;
       };