This introduces a new "controlled link" file type where the file is not
stored in Moodle - but Moodle will control the access permissions on the file.
Plugins can "freeze" a filearea which means Moodle will take ownership of all the remote
files of this type.
When accessing a file, if the "filebrowser" infomation indicates the current user can write to the file, they
will be granted temporary write access.
Part of MDL-58220
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.