Fix CamelCase problem in local_codechecker

https://tracker.moodle.org/browse/CONTRIB-5240

It does not break anything here (Mac), and should be fixing problems is other, case-sensitive, OSs.

Basically it does:

- The "moodle" standard to be 100% lowercase everywehre.
- The "PHPCompatibility" to be 100% camelcased everywhere.

That's all. Would be great to get it merged so we can verify if that solves recent CI problems.