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/windows/windows.go

@@ -38,6 +38,7 @@ import (
 	"github.com/containerd/containerd/mount"
 	"github.com/containerd/containerd/platforms"
 	"github.com/containerd/containerd/plugin"
+	"github.com/containerd/containerd/plugins"
 	"github.com/containerd/containerd/snapshots"
 	"github.com/containerd/containerd/snapshots/storage"
 	"github.com/containerd/continuity/fs"
@@ -47,7 +48,7 @@ import (
 
 func init() {
 	plugin.Register(&plugin.Registration{
-		Type: plugin.SnapshotPlugin,
+		Type: plugins.SnapshotPlugin,
 		ID:   "windows",
 		InitFn: func(ic *plugin.InitContext) (interface{}, error) {
 			ic.Meta.Platforms = []ocispec.Platform{platforms.DefaultSpec()}