release: use sets to select base packages

The introduction of package sets allows us to replace the current
fragile string matching with this simpler and more robust alternative.

Sponsored by: The FreeBSD Foundation
MFC after: 3 seconds
Reviewed by: emaste, ivy
Differential Revision: https://reviews.freebsd.org/D52592

(cherry picked from commit 6cdca18d79b9e247d8a34e266fe2215e4bfa1b05)