replace "dep" with "go mod"

A simple "go mod init && go mod tidy && go mod vendor" was enough and
mostly seems to have picked the same revisions of the dependencies as
before. Notable exceptions are klog and gogo/protobuf.

vendor/k8s.io/klog/klog_file.go

@@ -97,9 +97,11 @@ var onceLogDirs sync.Once
 // contains tag ("INFO", "FATAL", etc.) and t.  If the file is created
 // successfully, create also attempts to update the symlink for that tag, ignoring
 // errors.
-func create(tag string, t time.Time) (f *os.File, filename string, err error) {
+// The startup argument indicates whether this is the initial startup of klog.
+// If startup is true, existing files are opened for appending instead of truncated.
+func create(tag string, t time.Time, startup bool) (f *os.File, filename string, err error) {
 	if logging.logFile != "" {
-		f, err := os.Create(logging.logFile)
+		f, err := openOrCreate(logging.logFile, startup)
 		if err == nil {
 			return f, logging.logFile, nil
 		}
@@ -113,7 +115,7 @@ func create(tag string, t time.Time) (f *os.File, filename string, err error) {
 	var lastErr error
 	for _, dir := range logDirs {
 		fname := filepath.Join(dir, name)
-		f, err := os.Create(fname)
+		f, err := openOrCreate(fname, startup)
 		if err == nil {
 			symlink := filepath.Join(dir, link)
 			os.Remove(symlink)        // ignore err
@@ -124,3 +126,14 @@ func create(tag string, t time.Time) (f *os.File, filename string, err error) {
 	}
 	return nil, "", fmt.Errorf("log: cannot create log: %v", lastErr)
 }
+
+// The startup argument indicates whether this is the initial startup of klog.
+// If startup is true, existing files are opened for appending instead of truncated.
+func openOrCreate(name string, startup bool) (*os.File, error) {
+	if startup {
+		f, err := os.OpenFile(name, os.O_RDWR|os.O_CREATE|os.O_APPEND, 0666)
+		return f, err
+	}
+	f, err := os.Create(name)
+	return f, err
+}