Fix fatal error referencing PHP_Invoker (3.8-g5514f28)

During shutdown, PHPUnit tries to autoload PHP_Invoker which is not part of the composer.json file. This patch works around the issue by explicitly not having the autoloader called when `class_exists()` is used although there is probably a dependency not being resolved correctly as well.

There are a handful of other `class_exists` calls that do not disable the autoloader, they may also be in error.