I have directory
"app/libraries", // <<----- this one
You can try something which I recently tried and it's worked for me. Navigate to
vendor/composer/. Since you've said that you have one php file in libraries and it's working this means you will have it in followed files and you can copy/paste the lines and change with your files.
First open file
autoload_classmap.php and add at the bottom of the array
'yourfile' => $baseDir . '/app/libraries/yourfile.php',
Second open file
autoload_static.php and at the bottom add your file again
public static $classMap = array ( ... 'yourfile' => __DIR__. '/../..' . '/app/libraries/yourfile.php',
Save both files and reload. In my case this loaded the files without need of auto-dump.