vimUtils: represent vim plugins as derivations

Vim plugins were previously represented as strings by default,
necessitating a `knownPlugins` set. Backwards compatibility is kept
(strings are still accepted), so vam plugins should continue to work.

pkgs/misc/vim-plugins/update.py

@@ -310,6 +310,7 @@ def generate_nix(plugins: List[Tuple[str, str, Plugin]]):
                 f"""
   {plugin.normalized_name} = buildVimPluginFrom2Nix {{
     name = "{plugin.normalized_name}-{plugin.version}";
+    pname = "{plugin.normalized_name}";
     src = fetchFromGitHub {{
       owner = "{owner}";
       repo = "{repo}";