Move internal packages to an internal folder

For reference: https://go.dev/doc/go1.4#internalpackages

internal/logger/logger.go

@@ -0,0 +1,97 @@
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package logger // import "miniflux.app/v2/internal/logger"
+
+import (
+	"fmt"
+	"os"
+	"time"
+)
+
+var requestedLevel = InfoLevel
+var displayDateTime = false
+
+// LogLevel type.
+type LogLevel uint32
+
+const (
+	// FatalLevel should be used in fatal situations, the app will exit.
+	FatalLevel LogLevel = iota
+
+	// ErrorLevel should be used when someone should really look at the error.
+	ErrorLevel
+
+	// InfoLevel should be used during normal operations.
+	InfoLevel
+
+	// DebugLevel should be used only during development.
+	DebugLevel
+)
+
+func (level LogLevel) String() string {
+	switch level {
+	case DebugLevel:
+		return "DEBUG"
+	case InfoLevel:
+		return "INFO"
+	case ErrorLevel:
+		return "ERROR"
+	case FatalLevel:
+		return "FATAL"
+	default:
+		return "UNKNOWN"
+	}
+}
+
+// EnableDateTime enables date time in log messages.
+func EnableDateTime() {
+	displayDateTime = true
+}
+
+// EnableDebug increases logging, more verbose (debug)
+func EnableDebug() {
+	requestedLevel = DebugLevel
+	formatMessage(InfoLevel, "Debug mode enabled")
+}
+
+// Debug sends a debug log message.
+func Debug(format string, v ...interface{}) {
+	if requestedLevel >= DebugLevel {
+		formatMessage(DebugLevel, format, v...)
+	}
+}
+
+// Info sends an info log message.
+func Info(format string, v ...interface{}) {
+	if requestedLevel >= InfoLevel {
+		formatMessage(InfoLevel, format, v...)
+	}
+}
+
+// Error sends an error log message.
+func Error(format string, v ...interface{}) {
+	if requestedLevel >= ErrorLevel {
+		formatMessage(ErrorLevel, format, v...)
+	}
+}
+
+// Fatal sends a fatal log message and stop the execution of the program.
+func Fatal(format string, v ...interface{}) {
+	if requestedLevel >= FatalLevel {
+		formatMessage(FatalLevel, format, v...)
+		os.Exit(1)
+	}
+}
+
+func formatMessage(level LogLevel, format string, v ...interface{}) {
+	var prefix string
+
+	if displayDateTime {
+		prefix = fmt.Sprintf("[%s] [%s] ", time.Now().Format("2006-01-02T15:04:05"), level)
+	} else {
+		prefix = fmt.Sprintf("[%s] ", level)
+	}
+
+	fmt.Fprintf(os.Stderr, prefix+format+"\n", v...)
+}