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/devmapper/plugin/plugin.go

@@ -24,12 +24,13 @@ 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/devmapper"
 )
 
 func init() {
-	plugin.Register(&plugin.Registration{
+	registry.Register(&plugin.Registration{
 		Type:   plugins.SnapshotPlugin,
 		ID:     "devmapper",
 		Config: &devmapper.Config{},
@@ -46,7 +47,7 @@ func init() {
 			}
 
 			if config.RootPath == "" {
-				config.RootPath = ic.Root
+				config.RootPath = ic.Properties[plugins.PropertyRootDir]
 			}
 
 			return devmapper.NewSnapshotter(ic.Context, config)