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>

metadata/plugin/plugin.go

@@ -28,6 +28,7 @@ import (
 	"github.com/containerd/containerd/metadata"
 	"github.com/containerd/containerd/pkg/timeout"
 	"github.com/containerd/containerd/plugin"
+	"github.com/containerd/containerd/plugins"
 	"github.com/containerd/containerd/snapshots"
 	"github.com/containerd/log"
 
@@ -82,12 +83,12 @@ func (bc *BoltConfig) Validate() error {
 
 func init() {
 	plugin.Register(&plugin.Registration{
-		Type: plugin.MetadataPlugin,
+		Type: plugins.MetadataPlugin,
 		ID:   "bolt",
 		Requires: []plugin.Type{
-			plugin.ContentPlugin,
-			plugin.EventPlugin,
-			plugin.SnapshotPlugin,
+			plugins.ContentPlugin,
+			plugins.EventPlugin,
+			plugins.SnapshotPlugin,
 		},
 		Config: &BoltConfig{
 			ContentSharingPolicy: SharingPolicyShared,
@@ -96,12 +97,12 @@ func init() {
 			if err := os.MkdirAll(ic.Root, 0711); err != nil {
 				return nil, err
 			}
-			cs, err := ic.Get(plugin.ContentPlugin)
+			cs, err := ic.Get(plugins.ContentPlugin)
 			if err != nil {
 				return nil, err
 			}
 
-			snapshottersRaw, err := ic.GetByType(plugin.SnapshotPlugin)
+			snapshottersRaw, err := ic.GetByType(plugins.SnapshotPlugin)
 			if err != nil {
 				return nil, err
 			}
@@ -119,7 +120,7 @@ func init() {
 				snapshotters[name] = sn.(snapshots.Snapshotter)
 			}
 
-			ep, err := ic.Get(plugin.EventPlugin)
+			ep, err := ic.Get(plugins.EventPlugin)
 			if err != nil {
 				return nil, err
 			}