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/testing/prototest/prototest_test.go b/testing/prototest/prototest_test.go
index 5eec961..44fa0bc 100644
--- a/testing/prototest/prototest_test.go
+++ b/testing/prototest/prototest_test.go
@@ -15,7 +15,7 @@
 
 	irregularpb "google.golang.org/protobuf/internal/testprotos/irregular"
 	legacypb "google.golang.org/protobuf/internal/testprotos/legacy"
-	legacy1pb "google.golang.org/protobuf/internal/testprotos/legacy/proto2.v0.0.0-20160225-2fc053c5"
+	legacy1pb "google.golang.org/protobuf/internal/testprotos/legacy/proto2_20160225_2fc053c5"
 	testpb "google.golang.org/protobuf/internal/testprotos/test"
 	_ "google.golang.org/protobuf/internal/testprotos/test/weak1"
 	_ "google.golang.org/protobuf/internal/testprotos/test/weak2"