Move plugin type definitions to containerd plugins package

The plugins packages defines the plugins used by containerd.
Move all the types and properties to this package.

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/plugins"
 	"github.com/containerd/containerd/snapshots/devmapper"
 )
 
 func init() {
 	plugin.Register(&plugin.Registration{
-		Type:   plugin.SnapshotPlugin,
+		Type:   plugins.SnapshotPlugin,
 		ID:     "devmapper",
 		Config: &devmapper.Config{},
 		InitFn: func(ic *plugin.InitContext) (interface{}, error) {