Move aliases to compatbility shims.

Move the backwards compatibility aliases out of the bootstrap and into
stub files. By using stub files we don't force the aliased classes to be
eagerly loaded on every request. Instead they are only loaded when they
are used.

config/bootstrap.php

@@ -17,19 +17,6 @@ use Cake\Routing\Router;
 
 define('TIME_START', microtime(true));
 
-// @deprecated Backward compatibility with 2.x series
-if (PHP_VERSION_ID < 70000) {
-    class_alias('Cake\Utility\Text', 'Cake\Utility\String');
-}
-
-// @deprecated Backward compatibility with 2.x, 3.0.x
-class_alias('Cake\Mailer\AbstractTransport', 'Cake\Network\Email\AbstractTransport');
-class_alias('Cake\Mailer\Transport\DebugTransport', 'Cake\Network\Email\DebugTransport');
-class_alias('Cake\Mailer\Email', 'Cake\Network\Email\Email');
-class_alias('Cake\Mailer\Transport\MailTransport', 'Cake\Network\Email\MailTransport');
-class_alias('Cake\Mailer\Transport\SmtpTransport', 'Cake\Network\Email\SmtpTransport');
-
-
 require CAKE . 'basics.php';
 
 // Sets the initial router state so future reloads work.

src/Network/Email/AbstractTransport.php

@@ -0,0 +1,3 @@
+<?php
+// @deprecated Backward compatibility with 2.x, 3.0.x
+class_alias('Cake\Mailer\AbstractTransport', 'Cake\Network\Email\AbstractTransport');

src/Network/Email/DebugTransport.php

@@ -0,0 +1,3 @@
+<?php
+// @deprecated Backward compatibility with 2.x, 3.0.x
+class_alias('Cake\Mailer\Transport\DebugTransport', 'Cake\Network\Email\DebugTransport');

src/Network/Email/Email.php

@@ -0,0 +1,3 @@
+<?php
+// @deprecated Backward compatibility with 2.x, 3.0.x
+class_alias('Cake\Mailer\Email', 'Cake\Network\Email\Email');

src/Network/Email/MailTransport.php

@@ -0,0 +1,3 @@
+<?php
+// @deprecated Backward compatibility with 2.x, 3.0.x
+class_alias('Cake\Mailer\Transport\MailTransport', 'Cake\Network\Email\MailTransport');

src/Network/Email/SmtpTransport.php

@@ -0,0 +1,3 @@
+<?php
+// @deprecated Backward compatibility with 2.x, 3.0.x
+class_alias('Cake\Mailer\Transport\SmtpTransport', 'Cake\Network\Email\SmtpTransport');

src/Utility/String.php

@@ -0,0 +1,5 @@
+<?php
+// @deprecated Backward compatibility with 2.x series
+if (PHP_VERSION_ID < 70000) {
+    class_alias('Cake\Utility\Text', 'Cake\Utility\String');
+}