Improve performance on psr4 autoload file finding

src/Composer/Autoload/ClassLoader.php

@@ -374,9 +374,13 @@ class ClassLoader
 
         $first = $class[0];
         if (isset($this->prefixLengthsPsr4[$first])) {
-            foreach ($this->prefixLengthsPsr4[$first] as $prefix => $length) {
-                if (0 === strpos($class, $prefix)) {
-                    foreach ($this->prefixDirsPsr4[$prefix] as $dir) {
+            $subPath = $class;
+            while (false !== $lastPos = strrpos($subPath, '\\')) {
+                $subPath = substr($subPath, 0, $lastPos);
+                $search = $subPath.'\\';
+                if (isset($this->prefixDirsPsr4[$search])) {
+                    foreach ($this->prefixDirsPsr4[$search] as $dir) {
+                        $length = $this->prefixLengthsPsr4[$first][$search];
                         if (file_exists($file = $dir . DIRECTORY_SEPARATOR . substr($logicalPathPsr4, $length))) {
                             return $file;
                         }