pkgbase: collapse -PRERELEASE into STABLE/CURRENT case for version suffix

-PRERELEASE exists as a given stable/X branch while the next minor version
release is in progress. From a functional standpoint, it should be treated
as -STABLE since it'll typically be a superset of what's included in the
concurrent releng branch.

Reviewed by: gjb
Differential Revision: https://reviews.freebsd.org/D28166