Prepend runtime-autoloaders so that dependencies get loaded over Composer bundled packages

As Composer is generally compatible with newer versions of Symfony than the bundled ones, this should
not be a problem if dependencies are shared, and it ensures that the most relevant code gets loaded.

src/Composer/Autoload/AutoloadGenerator.php

@@ -517,9 +517,9 @@ EOF;
      * @param  array       $autoloads see parseAutoloads return value
      * @return ClassLoader
      */
-    public function createLoader(array $autoloads)
+    public function createLoader(array $autoloads, $vendorDir = null)
     {
-        $loader = new ClassLoader();
+        $loader = new ClassLoader($vendorDir);
 
         if (isset($autoloads['psr-0'])) {
             foreach ($autoloads['psr-0'] as $namespace => $path) {

src/Composer/EventDispatcher/EventDispatcher.php

@@ -435,8 +435,8 @@ class EventDispatcher
         $packages = $this->composer->getRepositoryManager()->getLocalRepository()->getCanonicalPackages();
         $packageMap = $generator->buildPackageMap($this->composer->getInstallationManager(), $package, $packages);
         $map = $generator->parseAutoloads($packageMap, $package);
-        $this->loader = $generator->createLoader($map);
-        $this->loader->register();
+        $this->loader = $generator->createLoader($map, $this->composer->getConfig()->get('vendor-dir'));
+        $this->loader->register(true);
 
         return $scripts[$event->getName()];
     }

src/Composer/Plugin/PluginManager.php

@@ -198,8 +198,8 @@ class PluginManager
         }
 
         $map = $generator->parseAutoloads($autoloads, $rootPackage);
-        $classLoader = $generator->createLoader($map);
-        $classLoader->register();
+        $classLoader = $generator->createLoader($map, $this->composer->getConfig()->get('vendor-dir'));
+        $classLoader->register(true);
 
         foreach ($classes as $class) {
             if (class_exists($class, false)) {