Allow to include a PHP code file

Summary:
This commit is part of an effort to reduce the -er classes.

The methods from the Autoloader class to include a file
more naturally belong to a class representing a code file.

Test Plan:

• Unit tests still run for Registration
• New unit tests for the class

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1620