plugins/completion: move to by-name

- Move nvim-cmp to plugins/cmp
- Move other completion plugins to plugins/by-name

plugins/completion/cmp/sources/cmp-tabnine.nix

@@ -1,19 +0,0 @@
-{
-  lib,
-  helpers,
-  config,
-  ...
-}:
-with lib;
-let
-  cfg = config.plugins.cmp-tabnine;
-in
-{
-  options.plugins.cmp-tabnine = helpers.neovim-plugin.extraOptionsOptions;
-
-  config = mkIf cfg.enable {
-    extraConfigLua = ''
-      require('cmp_tabnine.config'):setup(${helpers.toLuaObject cfg.extraOptions})
-    '';
-  };
-}