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.
New PARAM_COMPONENT, PARAM_AREA and PARAM_PLUGIN + fixing of hopefully all current incorrect parameter types. This should help with diagnosing of incorrectly named 3rd party plugins too.