MDLSITE-2216 Prevent debugging in deprecated call

get_core_subsystems() now deprecated, does no support short paths
anymore. This commit just change the call to ask for full paths
always to avoid a debugging() message. Note we don't change to the
new core_component::get_subsystems() because that would lead the
moodlecheck to only work against 2.6 and upwards. So we continue
using the deprecated call.