Fix hetjob component incorrect Priority state reason at allocation time.

_top_priority() is used to determine at job allocation time (called from
job_allocate()) if any other job has a higher priority than the specified
job being allocated. If so, it sets the job state_reason to WAIT_PRIORITY.
This patch skips the priority comparison if the two jobs being compared
are pack job components from the same heterogeneous job.

Bug 4400.