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>

services/snapshots/service.go

@@ -24,6 +24,7 @@ import (
 	"github.com/containerd/containerd/errdefs"
 	"github.com/containerd/containerd/mount"
 	"github.com/containerd/containerd/plugin"
+	"github.com/containerd/containerd/plugins"
 	ptypes "github.com/containerd/containerd/protobuf/types"
 	"github.com/containerd/containerd/services"
 	"github.com/containerd/containerd/snapshots"
@@ -33,10 +34,10 @@ import (
 
 func init() {
 	plugin.Register(&plugin.Registration{
-		Type: plugin.GRPCPlugin,
+		Type: plugins.GRPCPlugin,
 		ID:   "snapshots",
 		Requires: []plugin.Type{
-			plugin.ServicePlugin,
+			plugins.ServicePlugin,
 		},
 		InitFn: newService,
 	})
@@ -50,11 +51,11 @@ type service struct {
 }
 
 func newService(ic *plugin.InitContext) (interface{}, error) {
-	plugins, err := ic.GetByType(plugin.ServicePlugin)
+	sps, err := ic.GetByType(plugins.ServicePlugin)
 	if err != nil {
 		return nil, err
 	}
-	p, ok := plugins[services.SnapshotsService]
+	p, ok := sps[services.SnapshotsService]
 	if !ok {
 		return nil, errors.New("snapshots service not found")
 	}

services/snapshots/snapshotters.go

@@ -19,18 +19,19 @@ package snapshots
 import (
 	"github.com/containerd/containerd/metadata"
 	"github.com/containerd/containerd/plugin"
+	"github.com/containerd/containerd/plugins"
 	"github.com/containerd/containerd/services"
 )
 
 func init() {
 	plugin.Register(&plugin.Registration{
-		Type: plugin.ServicePlugin,
+		Type: plugins.ServicePlugin,
 		ID:   services.SnapshotsService,
 		Requires: []plugin.Type{
-			plugin.MetadataPlugin,
+			plugins.MetadataPlugin,
 		},
 		InitFn: func(ic *plugin.InitContext) (interface{}, error) {
-			m, err := ic.Get(plugin.MetadataPlugin)
+			m, err := ic.Get(plugins.MetadataPlugin)
 			if err != nil {
 				return nil, err
 			}