internal/testprotos/legacy: rename and regenerate

Avoid dots and dashes in the directory to avoid issues on
build systems that cannot support them well.

Change-Id: I7ea5e6ce0b16c7158c7e53bcf5c3c1a334fe4718
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/214342
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/internal/impl/extension_test.go b/internal/impl/extension_test.go
index 707cfeb..a8e2b68 100644
--- a/internal/impl/extension_test.go
+++ b/internal/impl/extension_test.go
@@ -8,10 +8,11 @@
 	"fmt"
 	"testing"
 
-	cmp "github.com/google/go-cmp/cmp"
-	testpb "google.golang.org/protobuf/internal/testprotos/test"
+	"github.com/google/go-cmp/cmp"
 	"google.golang.org/protobuf/proto"
 	pref "google.golang.org/protobuf/reflect/protoreflect"
+
+	testpb "google.golang.org/protobuf/internal/testprotos/test"
 )
 
 func TestExtensionType(t *testing.T) {