internal/impl: use current proto package, not old one

Correct a stray reference to github.com/golang/protobuf.

Updates golang/protobuf#962

Change-Id: I4a5f75d0aee61d28f18919a3ea728dc7a726da62
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/198998
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

internal/impl/extension_test.go

@@ -8,9 +8,9 @@ import (
 	"fmt"
 	"testing"
 
-	"github.com/golang/protobuf/proto"
 	cmp "github.com/google/go-cmp/cmp"
 	testpb "google.golang.org/protobuf/internal/testprotos/test"
+	"google.golang.org/protobuf/proto"
 	pref "google.golang.org/protobuf/reflect/protoreflect"
 )