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/message_reflect_test.go b/internal/impl/message_reflect_test.go
index bb6bda5..26c271d 100644
--- a/internal/impl/message_reflect_test.go
+++ b/internal/impl/message_reflect_test.go
@@ -22,7 +22,7 @@
 	pref "google.golang.org/protobuf/reflect/protoreflect"
 	"google.golang.org/protobuf/reflect/protoregistry"
 
-	proto2_20180125 "google.golang.org/protobuf/internal/testprotos/legacy/proto2.v1.0.0-20180125-92554152"
+	proto2_20180125 "google.golang.org/protobuf/internal/testprotos/legacy/proto2_20180125_92554152"
 	testpb "google.golang.org/protobuf/internal/testprotos/test"
 	"google.golang.org/protobuf/types/descriptorpb"
 )
@@ -968,7 +968,7 @@
 }
 
 var enumMessagesType = pimpl.MessageInfo{GoReflectType: reflect.TypeOf(new(EnumMessages)), Desc: mustMakeMessageDesc("enum-messages.proto", pref.Proto2, `
-		dependency: ["enum2.proto", "enum3.proto", "scalar2.proto", "scalar3.proto", "proto2.v1.0.0-20180125-92554152/test.proto"]
+		dependency: ["enum2.proto", "enum3.proto", "scalar2.proto", "scalar3.proto", "proto2_20180125_92554152/test.proto"]
 	`, `
 		name: "EnumMessages"
 		field: [