Re-vendor, primarily for https://github.com/containers/storage/pull/11

containers/storage got new dependencies, so we will need to re-vendor
eventually anyway, and having this separate from other major work is
cleaner.

But the primary goal of this commit is to see whether it makes skopeo
buildable on OS X.

vendor/github.com/docker/distribution/README.md

@@ -76,8 +76,7 @@ may be the better choice.
 For those who have previously deployed their own registry based on the Registry
 1.0 implementation and wish to deploy a Registry 2.0 while retaining images,
 data migration is required. A tool to assist with migration efforts has been
-created. For more information see [docker/migrator]
-(https://github.com/docker/migrator).
+created. For more information see [docker/migrator](https://github.com/docker/migrator).
 
 ## Contribute
 

vendor/github.com/docker/distribution/reference/reference.go

@@ -15,7 +15,7 @@
 //	tag                             := /[\w][\w.-]{0,127}/
 //
 //	digest                          := digest-algorithm ":" digest-hex
-//	digest-algorithm                := digest-algorithm-component [ digest-algorithm-separator digest-algorithm-component ]
+//	digest-algorithm                := digest-algorithm-component [ digest-algorithm-separator digest-algorithm-component ]*
 //	digest-algorithm-separator      := /[+.-_]/
 //	digest-algorithm-component      := /[A-Za-z][A-Za-z0-9]*/
 //	digest-hex                      := /[0-9a-fA-F]{32,}/ ; At least 128 bit digest value