Check autoload functions before callback

[barryvdh]

Check the autoload functions before and after. Unset the ones added during callback. Refs #11940

[barryvdh]

The return signature is changed in PHP8.0; https://www.php.net/manual/en/function.spl-autoload-functions.php

8.0.0 Return value was updated to always be an array; previously this function returned false if the autoload queue wasn't activated.

In our case we are pretty sure that it is set, but not sure what the best way is to make phpstan happy here for 7.2 and 8.1