repository: move Blob, Blobs and PackedBlob to pack package

This removes them from the public interface. The latter now only
provides the PackBlob interface, without being bound to the type used
internally by the pack package.

internal/repository/pack/packedblob.go

@@ -0,0 +1,19 @@
+package pack
+
+import "github.com/restic/restic/internal/restic"
+
+// PackedBlob is one index entry for a blob in a pack (may be duplicate across indexes).
+type PackedBlob struct {
+	Pack restic.ID
+	Blob Blob
+}
+
+func (pb *PackedBlob) PackID() restic.ID { return pb.Pack }
+
+func (pb *PackedBlob) Handle() restic.BlobHandle { return pb.Blob.BlobHandle }
+
+func (pb *PackedBlob) CiphertextLength() uint { return pb.Blob.Length }
+
+func (pb *PackedBlob) PlaintextLength() uint { return pb.Blob.DataLength() }
+
+func (pb *PackedBlob) IsCompressed() bool { return pb.Blob.IsCompressed() }