dirname() is a slow function compared with __DIR__ and using
'/../'.  Moodle has a large number of legacy files that are included
each time a page loads and is not able to use an autoloader as it is
functional code.  This allows those required includes to perform as
best as possible in this situation.

This commit refactors how associated glossary is searched for and removes
unnecessary DB queries. Also prevents from situations when the global glossary or course have
been deleted