Move manifest package to schema1

As we begin our march towards multi-arch, we must prepare for the reality of
multiple manifest schemas. This is the beginning of a set of changes to
facilitate this. We are both moving this package into its target position where
it may live peacefully next to other manfiest versions.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

manifest/versioned.go

@@ -0,0 +1,9 @@
+package manifest
+
+// Versioned provides a struct with just the manifest schemaVersion. Incoming
+// content with unknown schema version can be decoded against this struct to
+// check the version.
+type Versioned struct {
+	// SchemaVersion is the image manifest schema that this image follows
+	SchemaVersion int `json:"schemaVersion"`
+}