Hook additional autoloaders on post-autoload-dump

[aaemnnosttv]

Summary

This PR tweaks the "hooking" of our generated autoloaders on Composer's native autoload action.

This decouples the autoload generation from the dependency prefixing and ensures that all autoloaders are regenerated when invoking composer dump-autoload.

Addresses issue #612

Relevant technical choices

Checklist

• My code is tested and passes existing unit tests.
• My code has an appropriate set of unit tests which all pass.
• My code is backward-compatible with WordPress 4.7 and PHP 5.4.
• My code follows the WordPress coding standards.
• My code has proper inline documentation.
• I have signed the Contributor License Agreement (see https://cla.developers.google.com/).