Prevent class_exists() from calling autoloaders.

The update checker uses class_exists in several ways:

- As a guard clause around `class Whatever` definitions. This ensures we don't try to define a class that has already been loaded by a different plugin. In this case, autoloading is not necessary because we already know how to load the class. Also, we *want to* load our version of that class if possible - the version that gets loaded by somebody else's autoloader might be different and incompatible.
- As a guard clause before `require` statements that include a class. This is conceptually the same as the previous example. 
- To enable optional features if Debug Bar is active. The latest compatible version of Debug Bar doesn't use autoloading, so it would again be unnecessary in this case.

debug-bar-panel.php

@@ -1,6 +1,6 @@
 <?php
 
-if ( !class_exists('PluginUpdateCheckerPanel') && class_exists('Debug_Bar_Panel') ) {
+if ( !class_exists('PluginUpdateCheckerPanel', false) && class_exists('Debug_Bar_Panel', false) ) {
 
 /**
  * A Debug Bar panel for the plugin update checker.

debug-bar-plugin.php

@@ -1,5 +1,5 @@
 <?php
-if ( !class_exists('PucDebugBarPlugin') ) {
+if ( !class_exists('PucDebugBarPlugin', false) ) {
 
 class PucDebugBarPlugin {
 	/** @var PluginUpdateChecker */
@@ -23,7 +23,7 @@ class PucDebugBarPlugin {
 	 */
 	public function addDebugBarPanel($panels) {
 		require_once dirname(__FILE__) . '/debug-bar-panel.php';
-		if ( current_user_can('update_plugins') && class_exists('PluginUpdateCheckerPanel') ) {
+		if ( current_user_can('update_plugins') && class_exists('PluginUpdateCheckerPanel', false) ) {
 			$panels[] = new PluginUpdateCheckerPanel($this->updateChecker);
 		}
 		return $panels;

github-checker.php

@@ -1,6 +1,6 @@
 <?php
 
-if ( !class_exists('PucGitHubChecker_2_0') ):
+if ( !class_exists('PucGitHubChecker_2_0', false) ):
 
 class PucGitHubChecker_2_0 extends PluginUpdateChecker_2_0 {
 	/**
@@ -254,7 +254,7 @@ class PucGitHubChecker_2_0 extends PluginUpdateChecker_2_0 {
 	 * @return string
 	 */
 	protected function parseMarkdown($markdown) {
-		if ( !class_exists('Parsedown') ) {
+		if ( !class_exists('Parsedown', false) ) {
 			require_once(dirname(__FILE__) . '/vendor/Parsedown.php');
 		}
 
@@ -406,7 +406,7 @@ class PucGitHubChecker_2_0 extends PluginUpdateChecker_2_0 {
 	}
 
 	protected function parseReadme($content) {
-		if ( !class_exists('PucReadmeParser') ) {
+		if ( !class_exists('PucReadmeParser', false) ) {
 			require_once(dirname(__FILE__) . '/vendor/readme-parser.php');
 		}
 		$parser = new PucReadmeParser();

plugin-update-checker.php

@@ -7,7 +7,7 @@
  * Released under the  MIT license. See license.txt for details.
  */
 
-if ( !class_exists('PluginUpdateChecker_2_0') ):
+if ( !class_exists('PluginUpdateChecker_2_0', false) ):
 
 /**
  * A custom plugin update checker. 
@@ -845,7 +845,7 @@ class PluginUpdateChecker_2_0 {
 	 * Initialize the update checker Debug Bar plugin/add-on thingy.
 	 */
 	public function initDebugBarPanel() {
-		if ( class_exists('Debug_Bar') ) {
+		if ( class_exists('Debug_Bar', false) ) {
 			require_once dirname(__FILE__) . '/debug-bar-plugin.php';
 			$this->debugBarPlugin = new PucDebugBarPlugin($this);
 		}
@@ -854,7 +854,7 @@ class PluginUpdateChecker_2_0 {
 
 endif;
 
-if ( !class_exists('PluginInfo_2_0') ):
+if ( !class_exists('PluginInfo_2_0', false) ):
 
 /**
  * A container class for holding and transforming various plugin metadata.
@@ -984,7 +984,7 @@ class PluginInfo_2_0 {
 	
 endif;
 
-if ( !class_exists('PluginUpdate_2_0') ):
+if ( !class_exists('PluginUpdate_2_0', false) ):
 
 /**
  * A simple container class for holding information about an available update.
@@ -1100,7 +1100,7 @@ class PluginUpdate_2_0 {
 	
 endif;
 
-if ( !class_exists('PucFactory') ):
+if ( !class_exists('PucFactory', false) ):
 
 /**
  * A factory that builds instances of other classes from this library.
@@ -1201,14 +1201,14 @@ PucFactory::addVersion('PucGitHubChecker', 'PucGitHubChecker_2_0', '2.0');
  * Create non-versioned variants of the update checker classes. This allows for backwards
  * compatibility with versions that did not use a factory, and it simplifies doc-comments.
  */
-if ( !class_exists('PluginUpdateChecker') ) {
+if ( !class_exists('PluginUpdateChecker', false) ) {
 	class PluginUpdateChecker extends PluginUpdateChecker_2_0 { }
 }
 
-if ( !class_exists('PluginUpdate') ) {
+if ( !class_exists('PluginUpdate', false) ) {
 	class PluginUpdate extends PluginUpdate_2_0 {}
 }
 
-if ( !class_exists('PluginInfo') ) {
+if ( !class_exists('PluginInfo', false) ) {
 	class PluginInfo extends PluginInfo_2_0 {}
 }

vendor/readme-parser.php

@@ -237,7 +237,7 @@ Class PucReadmeParser {
 		$text = call_user_func( array( __CLASS__, 'code_trick' ), $text, $markdown ); // A better parser than Markdown's for: backticks -> CODE
 
 		if ( $markdown ) { // Parse markdown.
-			if ( !class_exists('Parsedown') ) {
+			if ( !class_exists('Parsedown', false) ) {
 				require_once(dirname(__FILE__) . '/Parsedown.php');
 			}
 			$instance = Parsedown::instance();