Revert "Issue #2400407: One more: Update composer/classloader to latest version."

This reverts commit 5823083e.

core/vendor/composer/ClassLoader.php

@@ -351,7 +351,7 @@ private function findFileWithExtension($class, $ext)
             foreach ($this->prefixLengthsPsr4[$first] as $prefix => $length) {
                 if (0 === strpos($class, $prefix)) {
                     foreach ($this->prefixDirsPsr4[$prefix] as $dir) {
-                        if (is_file($file = $dir . DIRECTORY_SEPARATOR . substr($logicalPathPsr4, $length))) {
+                        if (file_exists($file = $dir . DIRECTORY_SEPARATOR . substr($logicalPathPsr4, $length))) {
                             return $file;
                         }
                     }
@@ -361,7 +361,7 @@ private function findFileWithExtension($class, $ext)
 
         // PSR-4 fallback dirs
         foreach ($this->fallbackDirsPsr4 as $dir) {
-            if (is_file($file = $dir . DIRECTORY_SEPARATOR . $logicalPathPsr4)) {
+            if (file_exists($file = $dir . DIRECTORY_SEPARATOR . $logicalPathPsr4)) {
                 return $file;
             }
         }
@@ -380,7 +380,7 @@ private function findFileWithExtension($class, $ext)
             foreach ($this->prefixesPsr0[$first] as $prefix => $dirs) {
                 if (0 === strpos($class, $prefix)) {
                     foreach ($dirs as $dir) {
-                        if (is_file($file = $dir . DIRECTORY_SEPARATOR . $logicalPathPsr0)) {
+                        if (file_exists($file = $dir . DIRECTORY_SEPARATOR . $logicalPathPsr0)) {
                             return $file;
                         }
                     }
@@ -390,7 +390,7 @@ private function findFileWithExtension($class, $ext)
 
         // PSR-0 fallback dirs
         foreach ($this->fallbackDirsPsr0 as $dir) {
-            if (is_file($file = $dir . DIRECTORY_SEPARATOR . $logicalPathPsr0)) {
+            if (file_exists($file = $dir . DIRECTORY_SEPARATOR . $logicalPathPsr0)) {
                 return $file;
             }
         }