Reset core_component caches before asking for components

Now local_ci requires Moodle 2.6dev (201307104) so the new
core_component stuff is available always.

No matter of that we continue using the old (deprecated in 2.6
and upwards) get_plugin_types|subsystems|...() calls. Some day we'll
move to the new methods available within core_component.

But the key change here is that, via inheritance, we add a reset() method
able to clean the static caches built in the class, so the nasty hack of
forcing dirroot will continue working and the components will be returned
properly.

Personally I'm not happy with this solution (double-hack), but it's the fastest
one I've been able to achieve. In the future we should, somehow install
the site properly, ask for the list of components and store it into some "work"
directory so other jobs can reuse it. But not today. :-(