Update the reference parsing logic to match what common tools do today (e.g. Docker CLI)

Author: jcarter3

ociref/reference_test.go

@@ -363,6 +363,13 @@ var parseReferenceTests = []struct {
 		input:   "[fe80::1%@invalidzone]:5000/repo",
 		wantErr: `invalid reference syntax`,
 	},
+	{
+		input: "index.docker.io/foo",
+		wantRef: Reference{
+			Host:       "docker.io",
+			Repository: "library/foo",
+		},
+	},
 }
 
 func TestParseReference(t *testing.T) {