diff: rename differ to comparer

Remove combined interface and split implementations.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

diff/walking/plugin/plugin.go

@@ -0,0 +1,39 @@
+package plugin
+
+import (
+	"github.com/containerd/containerd/diff"
+	"github.com/containerd/containerd/diff/apply"
+	"github.com/containerd/containerd/diff/walking"
+	"github.com/containerd/containerd/metadata"
+	"github.com/containerd/containerd/platforms"
+	"github.com/containerd/containerd/plugin"
+)
+
+func init() {
+	plugin.Register(&plugin.Registration{
+		Type: plugin.DiffPlugin,
+		ID:   "walking",
+		Requires: []plugin.Type{
+			plugin.MetadataPlugin,
+		},
+		InitFn: func(ic *plugin.InitContext) (interface{}, error) {
+			md, err := ic.Get(plugin.MetadataPlugin)
+			if err != nil {
+				return nil, err
+			}
+
+			ic.Meta.Platforms = append(ic.Meta.Platforms, platforms.DefaultSpec())
+			cs := md.(*metadata.DB).ContentStore()
+
+			return diffPlugin{
+				Comparer: walking.NewWalkingDiff(cs),
+				Applier:  apply.NewFileSystemApplier(cs),
+			}, nil
+		},
+	})
+}
+
+type diffPlugin struct {
+	diff.Comparer
+	diff.Applier
+}