apidiff: ignore internal pack

The feature is added to apidiff which allows users to ignore internal package comparisons

Fixes golang/go#52864.

cmd/apidiff/main.go

@@ -18,6 +18,7 @@ import (
 var (
 	exportDataOutfile = flag.String("w", "", "file for export data")
 	incompatibleOnly  = flag.Bool("incompatible", false, "display only incompatible changes")
+	allowInternal     = flag.Bool("allow-internal", false, "allow apidiff to compare internal packages")
 )
 
 func main() {
@@ -59,9 +60,11 @@ func main() {
 		}
 		oldpkg := mustLoadOrRead(flag.Arg(0))
 		newpkg := mustLoadOrRead(flag.Arg(1))
-		// if the package is an internal package, ignore it
-		if isInternalPackage(oldpkg.Path()) || isInternalPackage(newpkg.Path()) {
-			return
+		if !*allowInternal {
+			if isInternalPackage(oldpkg.Path()) && isInternalPackage(newpkg.Path()) {
+				fmt.Fprintf(os.Stderr, "Ignoring internal package %s\n", oldpkg.Path())
+				os.Exit(0)
+			}
 		}
 		report := apidiff.Changes(oldpkg, newpkg)
 		var err error