Re-vendor, and use mtrmac/image-spec:id-based-loader to fix tests

Anyone running (vndr) currently ends up with failing tests in OCI schema
validation because gojsonschema has fixed its "$ref" interpretation, exposing
inconsistent URI usage inside image-spec/schema.

So, this runs (vndr), and uses mtrmac/image-spec:id-based-loader
( https://github.com/opencontainers/image-spec/pull/739 ) to make the tests pass
again.  As soon as that PR is merged we should revert to using the upstream
image-spec repo again.

vendor/golang.org/x/crypto/openpgp/keys.go

@@ -325,9 +325,8 @@ func ReadEntity(packets *packet.Reader) (*Entity, error) {
 		if e.PrivateKey, ok = p.(*packet.PrivateKey); !ok {
 			packets.Unread(p)
 			return nil, errors.StructuralError("first packet was not a public/private key")
-		} else {
-			e.PrimaryKey = &e.PrivateKey.PublicKey
 		}
+		e.PrimaryKey = &e.PrivateKey.PublicKey
 	}
 
 	if !e.PrimaryKey.PubKeyAlgo.CanSign() {