Don't try to autoload any of Composer's classes

If the composer autoloader has prepend-autoloader: false set, the PHPCS
autoloader will take priority over the composer autoloader, and composer
will have trouble autoloading its own autoloader.

Avoid all of that by simply not trying to load any classes that are
under the "Composer" namespace.

Fixes #1497

autoload.php

@@ -67,6 +67,12 @@ public static function load($class)
             // so this autoloader runs before the composer one as we need to include
             // all files so we can figure out what the class/interface/trait name is.
             if (self::$composerAutoloader === null) {
+                // Make sure we don't try to load any of Composer's classes
+                // while the autoloader is being setup.
+                if (strpos($class, 'Composer\\') === 0) {
+                    return;
+                }
+
                 if (strpos(__DIR__, 'phar://') !== 0
                     && file_exists(__DIR__.'/../../autoload.php') === true
                 ) {
@@ -82,7 +88,7 @@ public static function load($class)
                 } else {
                     self::$composerAutoloader = false;
                 }
-            }
+            }//end if
 
             $ds   = DIRECTORY_SEPARATOR;
             $path = false;