Early call to debugging() in core_component leads to fatal error

This is a regression caused by MDL-65646 when using third part plugins having subplugins with moodle 3.8dev (master) .

Basically we introduced some debugging() to tell developers that they third-part plugins should stop using subplugins.php and move to subplugins.json.

But that debugging is happening too early in Moodle's bootstrap/warmup process and debugging() may be not available yet.

Error follows:

[21-Jun-2019 08:46:16 UTC] Default exception handler: error/generalexceptionmessage Debug: 

Error code: generalexceptionmessage

$a contents: Call to undefined function debugging()

* line 540 of /lib/classes/component.php: Error thrown

* line 471 of /lib/classes/component.php: call to core_component::fetch_subtypes()

* line 383 of /lib/classes/component.php: call to core_component::fetch_plugintypes()

* line 357 of /lib/classes/component.php: call to core_component::fill_all_caches()

* line 293 of /lib/classes/component.php: call to core_component::get_cache_content()

* line 112 of /lib/classes/component.php: call to core_component::init()

* line ? of unknownfile: call to core_component::classloader()

* line 593 of /lib/setup.php: call to spl_autoload_call()

* line 57 of /config.php: call to require_once()

* line 30 of /index.php: call to require_once()

Note that, to be able to see the error above... we had to hack quite a bit because the exception handler itself was breaking (so early in the process). So MDL-65996 has been created about that.

Ciao

PS: Many thanks to daniss for raising the issue and helping to trace it down!

For posterity, the old first section of testing instructions:

A) Forcing the error within core.

1. Remove mod/assign/db/subplugins.json
2. Go to site main page
3. Verify you get a "debugging" error pointing to

   line 25 of /mod/assign/db/subplugins.php: Error thrown

   line 542 of /lib/classes/component.php: call to include()

   That's the importan point (the include one).

4. Verify that you get in the web server error logs the message "Use of subplugins.php has been deprecated. Please update your 'YOUR/PATH/TO/mod/assign' plugin to provide a subplugins.json file instead"