Split mail sender sub package from mailer service package (#32618)

Move all mail sender related codes into a sub package of
services/mailer. Just move, no code change.
Then we just have dependencies on go-mail package in the new sub
package. We can use other package to replace it because it's
unmaintainable. ref #18664

services/mailer/sender/sender.go

@@ -0,0 +1,27 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package sender
+
+import (
+	"code.gitea.io/gitea/modules/log"
+	"code.gitea.io/gitea/modules/setting"
+
+	"gopkg.in/gomail.v2"
+)
+
+type Sender gomail.Sender
+
+var Send = send
+
+func send(sender Sender, msgs ...*Message) error {
+	if setting.MailService == nil {
+		log.Error("Mailer: Send is being invoked but mail service hasn't been initialized")
+		return nil
+	}
+	goMsgs := []*gomail.Message{}
+	for _, msg := range msgs {
+		goMsgs = append(goMsgs, msg.ToMessage())
+	}
+	return gomail.Send(sender, goMsgs...)
+}