Refactor: libpacemaker: make transition.h internal

transition.h has always been installed even though it was purely internal.
Since libtransitioner is now merged into libpacemaker, move transition.h
to include/pcmki/pcmki_transition.h and make it internal.

Also, get rid of pcmki_sched_transition.h since it no longer contains
anything transition-related, and move its two function declarations to
pcmki_sched_utils.h.