internal/testprotos: hide all public testprotos

The encoding/testprotos and reflect/protoregistry/testprotos are
accessible by other modules. Move them under internal/testprotos
to dissuade programmers who are too lazy to use their own test protos
when they need one.

Change-Id: I3dbfbce74e68ef033ec252bed076861cb47dd21e
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/214341
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/encoding/protojson/encode_test.go b/encoding/protojson/encode_test.go
index 9ee9f25..2d46bd4 100644
--- a/encoding/protojson/encode_test.go
+++ b/encoding/protojson/encode_test.go
@@ -17,8 +17,8 @@
 	"google.golang.org/protobuf/proto"
 	preg "google.golang.org/protobuf/reflect/protoregistry"
 
-	"google.golang.org/protobuf/encoding/testprotos/pb2"
-	"google.golang.org/protobuf/encoding/testprotos/pb3"
+	pb2 "google.golang.org/protobuf/internal/testprotos/textpb2"
+	pb3 "google.golang.org/protobuf/internal/testprotos/textpb3"
 	"google.golang.org/protobuf/types/known/anypb"
 	"google.golang.org/protobuf/types/known/durationpb"
 	"google.golang.org/protobuf/types/known/emptypb"