MDL-81456 core: Use DI for all hook access

Using DI for all hook access means that it becomes significantly easier
to mock hooks and callbacks for unit testing without fundamentally
altering the structure of the code purely for the purposes of unit
testing.

lib/phpunit/bootstrap.php

@@ -262,6 +262,7 @@ if (PHPUNIT_UTIL) {
 
 // Make sure the hook manager gets initialised before anybody tries to override callbacks,
 // this is not using caches intentionally to help with development.
+// Note: We cannot use DI at this point in the bootstrap either.
 \core\hook\manager::get_instance();
 
 // Is database and dataroot ready for testing?