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/dummy.go

@@ -0,0 +1,26 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package sender
+
+import (
+	"bytes"
+	"io"
+
+	"code.gitea.io/gitea/modules/log"
+)
+
+// DummySender Sender sendmail mail sender
+type DummySender struct{}
+
+var _ Sender = &DummySender{}
+
+// Send send email
+func (s *DummySender) Send(from string, to []string, msg io.WriterTo) error {
+	buf := bytes.Buffer{}
+	if _, err := msg.WriteTo(&buf); err != nil {
+		return err
+	}
+	log.Debug("Mail From: %s To: %v Body: %s", from, to, buf.String())
+	return nil
+}