[Config] use is_file in FileResource::exists()

file resource existence check shouldn't return true if there's
directory with same name instead of file.

src/Symfony/Component/Config/Resource/FileResource.php

@@ -87,7 +87,7 @@ class FileResource implements ResourceInterface
      */
     public function exists()
     {
-        return file_exists($this->resource);
+        return is_file($this->resource);
     }
 
     /**