importer: refactor

- Use lease API (previoisly, GC was not supported)
- Refactored interfaces for ease of future Docker v1 importer support

For usage, please refer to `ctr images import --help`.

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

remotes/handlers.go

@@ -114,6 +114,7 @@ func fetch(ctx context.Context, ingester content.Ingester, fetcher Fetcher, desc
 func commitOpts(desc ocispec.Descriptor, r io.Reader) (io.Reader, []content.Opt) {
 	var childrenF func(r io.Reader) ([]ocispec.Descriptor, error)
 
+	// TODO(AkihiroSuda): use images/oci.GetChildrenDescriptors?
 	switch desc.MediaType {
 	case images.MediaTypeDockerSchema2Manifest, ocispec.MediaTypeImageManifest:
 		childrenF = func(r io.Reader) ([]ocispec.Descriptor, error) {