The classes that are being manually loaded have already been physically placed in the relevant location (/classes) for Moodle class autoloading to be performed, but named in a way that is not compliant with this.
There should be no need to come up with an alternate way for PHP classes to be loaded.
This code from https://github.com/turnitin/moodle-plagiarism_turnitin/blob/8cf0d4f8dbad46a36079c5f4b77ea294a1dcf8cc/lib.php#L70 onwards is anachronistic.
The classes that are being manually loaded have already been physically placed in the relevant location (/classes) for Moodle class autoloading to be performed, but named in a way that is not compliant with this.
There should be no need to come up with an alternate way for PHP classes to be loaded.