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.
Previously, any inappropriate whitespace found whilst building the admin
tree caused a JSON error and for the site administration tree retrieval to
fail.
This change ensures that access to the site administration tree is unbroken
for non-developers, whilst still alerting developers that there is an
issue.
