make DefaultLogger a variable instead of a type

Minor cleanup so that you can use `DefaultLogger` instead of
`DefaultLogger{}`.

event.go

@@ -371,7 +371,7 @@ func (l *EventListener) EnsureDefaults(logger Logger) {
 // specified logger.
 func MakeLoggingEventListener(logger Logger) EventListener {
 	if logger == nil {
-		logger = DefaultLogger{}
+		logger = DefaultLogger
 	}
 
 	return EventListener{

logger.go

@@ -16,16 +16,18 @@ type Logger interface {
 	Fatalf(format string, args ...interface{})
 }
 
+type defaultLogger struct{}
+
 // DefaultLogger logs to the Go stdlib logs.
-type DefaultLogger struct{}
+var DefaultLogger defaultLogger
 
 // Infof implements the Logger.Infof interface.
-func (DefaultLogger) Infof(format string, args ...interface{}) {
+func (defaultLogger) Infof(format string, args ...interface{}) {
 	_ = log.Output(2, fmt.Sprintf(format, args...))
 }
 
 // Fatalf implements the Logger.Fatalf interface.
-func (DefaultLogger) Fatalf(format string, args ...interface{}) {
+func (defaultLogger) Fatalf(format string, args ...interface{}) {
 	_ = log.Output(2, fmt.Sprintf(format, args...))
 	os.Exit(1)
 }

options.go

@@ -394,7 +394,7 @@ func (o *Options) EnsureDefaults() *Options {
 		}
 	}
 	if o.Logger == nil {
-		o.Logger = DefaultLogger{}
+		o.Logger = DefaultLogger
 	}
 	o.EventListener.EnsureDefaults(o.Logger)
 	if o.MaxManifestFileSize == 0 {