vim_configurable: handle python3Dependencies

pkgs/misc/vim-plugins/vim-utils.nix

@@ -3,6 +3,7 @@
 , nix-prefetch-hg, nix-prefetch-git
 , fetchFromGitHub, runtimeShell
 , hasLuaModule
+, python3
 }:
 
 /*
@@ -210,13 +211,24 @@ let
       # and can simply pass `null`.
       depsOfOptionalPlugins = lib.subtractLists opt (findDependenciesRecursively opt);
       startWithDeps = findDependenciesRecursively start;
+      allPlugins = lib.unique (startWithDeps ++ depsOfOptionalPlugins);
+      python3Env = python3.withPackages (ps:
+        lib.flatten (builtins.map (plugin: (plugin.python3Dependencies or (_: [])) ps) allPlugins)
+      );
     in
       [ "mkdir -p $out/pack/${packageName}/start" ]
       # To avoid confusion, even dependencies of optional plugins are added
       # to `start` (except if they are explicitly listed as optional plugins).
-      ++ (builtins.map (x: link x packageName "start") (lib.unique (startWithDeps ++ depsOfOptionalPlugins)))
+      ++ (builtins.map (x: link x packageName "start") allPlugins)
       ++ ["mkdir -p $out/pack/${packageName}/opt"]
       ++ (builtins.map (x: link x packageName "opt") opt)
+      # Assemble all python3 dependencies into a single `site-packages` to avoid doing recursive dependency collection
+      # for each plugin.
+      # This directory is only for python import search path, and will not slow down the startup time.
+      ++ [
+        "mkdir -p $out/pack/${packageName}/start/__python3_dependencies"
+        "ln -s ${python3Env}/${python3Env.sitePackages} $out/pack/${packageName}/start/__python3_dependencies/python3"
+      ]
     );
     packDir = (packages:
       stdenv.mkDerivation {