converter: make backend package independent

Move `pkg/converter/backend` to `pkg/backend`.

The advantage of this change is that if a component imports the
converter package, but it doesn't need backend support, the non-direct
dependencies in `pkg/backend` don't have to be imported either.

Signed-off-by: Yan Song <imeoer@linux.alibaba.com>

pkg/converter/types.go

@@ -7,8 +7,9 @@
 package converter
 
 import (
+	"context"
+
 	"github.com/containerd/containerd/content"
-	"github.com/containerd/nydus-snapshotter/pkg/converter/backend"
 	"github.com/opencontainers/go-digest"
 )
 
@@ -19,6 +20,19 @@ type Layer struct {
 	ReaderAt content.ReaderAt
 }
 
+// Backend uploads blobs generated by nydus-image builder to a backend storage such as:
+// - oss: A object storage backend, which uses its SDK to upload blob file.
+type Backend interface {
+	// Push pushes specified blob file to remote storage backend.
+	Push(ctx context.Context, ra content.ReaderAt, blobDigest digest.Digest) error
+	// Check checks whether a blob exists in remote storage backend,
+	// blob exists -> return (blobPath, nil)
+	// blob not exists -> return ("", err)
+	Check(blobDigest digest.Digest) (string, error)
+	// Type returns backend type name.
+	Type() string
+}
+
 type PackOption struct {
 	// WorkDir is used as the work directory during layer pack.
 	WorkDir string
@@ -34,7 +48,7 @@ type PackOption struct {
 	// Compressor specifies nydus blob compression algorithm.
 	Compressor string
 	// Backend uploads blobs generated by nydus-image builder to a backend storage.
-	Backend backend.Backend
+	Backend Backend
 }
 
 type MergeOption struct {