image: Rename 'descriptor' -> 'Descriptor' to make public

I'll need access to this type from the coming refs sub-package.
Pulling it out into its own subpackage avoids an:

  image -(need refs.Engine)-> refs -(need Descriptor)-> image

import cycle.  I've left the descriptor validation in the image
package, because it's high-level stuff that will depend on refs.

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

image/descriptor.go

@@ -23,14 +23,11 @@ import (
 	"os"
 	"path/filepath"
 
+	"github.com/opencontainers/image-spec/image/structure"
 	"github.com/pkg/errors"
 )
 
-type descriptor struct {
-	MediaType string `json:"mediaType"`
-	Digest    string `json:"digest"`
-	Size      int64  `json:"size"`
-}
+type descriptor structure.Descriptor
 
 func findDescriptor(w walker, name string) (*descriptor, error) {
 	var d descriptor

image/structure/descriptor.go

@@ -0,0 +1,22 @@
+// Copyright 2016 The Linux Foundation
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package structure
+
+// Descriptor represents application/vnd.oci.descriptor.v1.
+type Descriptor struct {
+	MediaType string `json:"mediaType"`
+	Digest    string `json:"digest"`
+	Size      int64  `json:"size"`
+}

image/structure/doc.go

@@ -0,0 +1,16 @@
+// Copyright 2016 The Linux Foundation
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package structure defines stucts for image-spec content.
+package structure