Mail: Make sure the `SMTP` class is only required once if a plugin requires `wp-includes/class-smtp.php` directly.

Follow-up to [48033], [48530].

Props oellin, greatsaltlake, audrasjb.
Fixes #52369. See #50716, #41750.
Built from https://develop.svn.wordpress.org/trunk@50022


git-svn-id: http://core.svn.wordpress.org/trunk@49723 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-smtp.php

@@ -10,6 +10,6 @@ _deprecated_file(
 	__( 'The SMTP class has been moved to the wp-includes/PHPMailer subdirectory and now uses the PHPMailer\PHPMailer namespace.' )
 );
 
-require __DIR__ . '/PHPMailer/SMTP.php';
+require_once __DIR__ . '/PHPMailer/SMTP.php';
 
 class_alias( PHPMailer\PHPMailer\SMTP::class, 'SMTP' );

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.7-alpha-50021';
+$wp_version = '5.7-alpha-50022';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.