Automatically importing module-provided translations during installation
Per the documentation:
https://api.drupal.org/api/drupal/core%21modules%21locale%21locale.api.php/group/interface_translation_properties/8.4.x


  Streamwrappers can be used in the server pattern definition. The interface translations directory (Configuration > Media > File system) can be addressed using the "translations://" streamwrapper. But also other streamwrappers can be used.


There are several examples given for valid server pattern definitions using different stream wrappers.

EDIT

I see what you mean about not wanting to use the translations:// stream wrapper.

Have you tried using the pattern definition exactly as given in the answer you cited?

modules/custom/example_module/translations/%language.po

If you do that, you should also take the name of your module out and just name the file fr.po.

Then make sure that you update translations with Drush:

drush locale-check &amp;&amp; drush locale-update &amp;&amp; drush cr

If that doesn't work, hopefully someone else can help, because I'm stumped.