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

routers/private/mail.go

@@ -17,6 +17,7 @@ import (
 	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/services/context"
 	"code.gitea.io/gitea/services/mailer"
+	sender_service "code.gitea.io/gitea/services/mailer/sender"
 )
 
 // SendEmail pushes messages to mail queue
@@ -81,7 +82,7 @@ func SendEmail(ctx *context.PrivateContext) {
 
 func sendEmail(ctx *context.PrivateContext, subject, message string, to []string) {
 	for _, email := range to {
-		msg := mailer.NewMessage(email, subject, message)
+		msg := sender_service.NewMessage(email, subject, message)
 		mailer.SendAsync(msg)
 	}