neovim: generate remote plugin manifest

This makes sure the user doesn't have to call `UpdateRemotePlugins`
manually for plugins installed through nix. A minor patch to neovim is
necessary, but it should be harmless. See
https://github.com/neovim/neovim/issues/9413 for a discussion about
the patch.

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

@@ -486,4 +486,12 @@ rec {
       });
     vimrcConfig.vam.pluginDictionaries = [ { names = [ "vim-trailing-whitespace" ]; } ];
   };
+
+  # system remote plugin manifest should be generated, deoplete should be usable
+  # without the user having to do `UpdateRemotePlugins`. To test, launch neovim
+  # and do `:call deoplete#enable()`. It will print an error if the remote
+  # plugin is not registered.
+  test_nvim_with_remote_plugin = neovim.override {
+    configure.pathogen.pluginNames = with vimPlugins; [ deoplete-nvim ];
+  };
 }