internal/version: move version information to internal package

Change-Id: I947876de5d290cf783d9ba798871725e77e16517
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/223277
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/integration_test.go b/integration_test.go
index e4444a4..2a9a470 100644
--- a/integration_test.go
+++ b/integration_test.go
@@ -25,7 +25,7 @@
 	"testing"
 	"time"
 
-	"google.golang.org/protobuf/runtime/protoimpl"
+	"google.golang.org/protobuf/internal/version"
 )
 
 var (
@@ -321,7 +321,7 @@
 	}
 	if *buildRelease {
 		t.Run("BuildRelease", func(t *testing.T) {
-			v := protoimpl.VersionString()
+			v := version.String()
 			for _, goos := range []string{"linux", "darwin", "windows"} {
 				for _, goarch := range []string{"386", "amd64"} {
 					binPath := filepath.Join("bin", fmt.Sprintf("protoc-gen-go.%v.%v.%v", v, goos, goarch))