Remove digest package's dependency on external sha implementation

The change relies on a refactor of the upstream resumable sha256/sha512 package
that opts to register implementations with the standard library. This allows
the resumable support to be detected where it matters, avoiding unnecessary and
complex code. It also ensures that consumers of the digest package don't need
to depend on the forked sha implementations.

We also get an optimization with this change. If the size of data written to a
digester is the same as the file size, we check to see if the digest has been
verified. This works if the blob is written and committed in a single request.

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

docs/storage/blobwriter_nonresumable.go

@@ -2,5 +2,16 @@
 
 package storage
 
-func (bw *blobWriter) setupResumableDigester() {
+import (
+	"github.com/docker/distribution/context"
+)
+
+// resumeHashAt is a noop when resumable digest support is disabled.
+func (bw *blobWriter) resumeDigestAt(ctx context.Context, offset int64) error {
+	return errResumableDigestNotAvailable
+}
+
+// storeHashState is a noop when resumable digest support is disabled.
+func (bw *blobWriter) storeHashState(ctx context.Context) error {
+	return errResumableDigestNotAvailable
 }