Use reference package internally

Most places in the registry were using string types to refer to
repository names. This changes them to use reference.Named, so the type
system can enforce validation of the naming rules.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

registry.go

@@ -2,6 +2,7 @@ package distribution
 
 import (
 	"github.com/docker/distribution/context"
+	"github.com/docker/distribution/reference"
 )
 
 // Scope defines the set of items that match a namespace.
@@ -32,7 +33,7 @@ type Namespace interface {
 	// Repository should return a reference to the named repository. The
 	// registry may or may not have the repository but should always return a
 	// reference.
-	Repository(ctx context.Context, name string) (Repository, error)
+	Repository(ctx context.Context, name reference.Named) (Repository, error)
 
 	// Repositories fills 'repos' with a lexigraphically sorted catalog of repositories
 	// up to the size of 'repos' and returns the value 'n' for the number of entries
@@ -49,7 +50,7 @@ type ManifestServiceOption interface {
 // Repository is a named collection of manifests and layers.
 type Repository interface {
 	// Name returns the name of the repository.
-	Name() string
+	Name() reference.Named
 
 	// Manifests returns a reference to this repository's manifest service.
 	// with the supplied options applied.