Do not autoload wp-config.php

Currently `wp-config.php` is autoloaded via composer's autoload files feature. This is included prior to the `altis.loaded_autoloader` hook which is when `.config/load.php` is included.

The end result is that default constants set in `wp-config-production.php` on our stacks can never be overridden if needed and other workarounds using filters are required.

We should load `wp-config.php` intentionally on the `altis.loaded_autoloader` hook with a higher priority than `.config/load.php`.

cc @stuartshields @nathanielks @Japh