Using 3rd party libraries I'm trying to load https://github.com/PHPOffice/PHPExcel in a custom module.  I've successfully added the library dependency to the composer.json file and Composer correctly downloads it into the 'vendor' folder.

The PHPExcel library doesn't use namespaces.  How do I access the library?  The following line

PHPExcel_IOFactory::load($excel_filepath);

results in 

Fatal error: Class 'Drupal\mymodule\Form\PHPExcel_IOFactory' not found in /home/me/webs/client/myhomedir/modules/custom/mymodule/src/Form/ImportProductsForm.php on line 60

If possible, I'd prefer not to use the Libraries module.
You are in your own namespace, that means PHP by default tries to load the class from your current namespace.

To load a non-namespaced class, you need to explicitly refer to the global namespace with a :

\PHPExcel_IOFactory::load($excel_filepath);