Refactor: libcrmcommon: New pcmk__map_element_name()

Only useful for mapping PCMKXE_PROMOTABLE_LEGACY to PCMKXE_CLONE, so
drop at 3.0.0 unless we come up with another use for it.

Also avoid pcmk__map_element_name() in contexts where the resource is
guaranteed not to be a clone.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>