deprecated: go-plugin library as runtime plugin

We, containerd, suggest user to use binary plugins or RPC-based plugins.
Since go plugin has too many restrictions, I'm not sure that how many users
use the go plugin to extend the core function in the production.

Based on the fact that we put a lot of effort to make external plugins
better, suggest to deprecate go-plugin type plugin in v2.0 and remove it
in v2.1

REF: https://github.com/containerd/containerd/pull/556

Signed-off-by: Wei Fu <fuweid89@gmail.com>

services/server/server.go

@@ -431,10 +431,11 @@ func (s *Server) Wait() {
 // of all plugins.
 func LoadPlugins(ctx context.Context, config *srvconfig.Config) ([]plugin.Registration, error) {
 	// load all plugins into containerd
-	path := config.PluginDir
+	path := config.PluginDir //nolint: staticcheck
 	if path == "" {
 		path = filepath.Join(config.Root, "plugins")
 	}
+	log.G(ctx).Warning("`go_plugin` is deprecated, please use `external plugins` instead")
 	if err := dynamic.Load(path); err != nil {
 		return nil, err
 	}