Allow self-registration to work outside Composer too

Summary:
Unit test for self-registration only checked we add an autoloader
function, but didn't test the correctness of the autoloader.

The autoloader worked fine if an autoloader is already there,
but if not will complain about missing classes.

Test Plan: Tested without vendor/ folder

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2546