exporter: support unpack opt for image exporter

It is enhancement which allows to unpack image into containerd
snapshotter storage by `--output type=image,<.>=<.>,unpack=true`.

In order to support this feature, we needs to extend the Snapshotter
witwh `Name() string` function. Because we needs to set gc label for
snapshotter which need snapshotter name.

fix: #908

Signed-off-by: Wei Fu <fuweid89@gmail.com>

source/http/httpsource_test.go

@@ -315,7 +315,7 @@ func newHTTPSource(tmpdir string) (source.Source, error) {
 	}
 
 	cm, err := cache.NewManager(cache.ManagerOpt{
-		Snapshotter:   snapshot.FromContainerdSnapshotter(snapshotter, nil),
+		Snapshotter:   snapshot.FromContainerdSnapshotter("native", snapshotter, nil),
 		MetadataStore: md,
 	})
 	if err != nil {