Generalize the plugin package

Remove containerd specific parts of the plugin package to prepare its
move out of the main repository. Separate the plugin registration
singleton into a separate package.

Separating out the plugin package and registration makes it easier to
implement external plugins without creating a dependency loop.

Signed-off-by: Derek McGowan <derek@mcg.dev>

snapshots/blockfile/plugin/plugin.go

@@ -21,6 +21,7 @@ import (
 
 	"github.com/containerd/containerd/platforms"
 	"github.com/containerd/containerd/plugin"
+	"github.com/containerd/containerd/plugin/registry"
 	"github.com/containerd/containerd/plugins"
 	"github.com/containerd/containerd/snapshots/blockfile"
 )
@@ -45,7 +46,7 @@ type Config struct {
 }
 
 func init() {
-	plugin.Register(&plugin.Registration{
+	registry.Register(&plugin.Registration{
 		Type:   plugins.SnapshotPlugin,
 		ID:     "blockfile",
 		Config: &Config{},
@@ -58,7 +59,7 @@ func init() {
 			}
 
 			var opts []blockfile.Opt
-			root := ic.Root
+			root := ic.Properties[plugins.PropertyRootDir]
 			if len(config.RootPath) != 0 {
 				root = config.RootPath
 			}