all: change module to google.golang.org/protobuf

Temporarily remove go.mod, since we can't generate an accurate one until
the corresponding v1 change is submitted.

Change-Id: I1e1ad97f2b455e33f61ffaeb8676289795e47e72
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/177000
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/internal/impl/legacy_extension.go b/internal/impl/legacy_extension.go
index 7a8fea7..eac04b9 100644
--- a/internal/impl/legacy_extension.go
+++ b/internal/impl/legacy_extension.go
@@ -7,8 +7,8 @@
 import (
 	"reflect"
 
-	pref "github.com/golang/protobuf/v2/reflect/protoreflect"
-	piface "github.com/golang/protobuf/v2/runtime/protoiface"
+	pref "google.golang.org/protobuf/reflect/protoreflect"
+	piface "google.golang.org/protobuf/runtime/protoiface"
 )
 
 func makeLegacyExtensionFieldsFunc(t reflect.Type) func(p *messageDataType) pref.KnownFields {