pkg: finish cleanup of install/plist targets
pkg: finish cleanup of install/plist targets

No functional changes; refactored for readability and exclusive
use of ".link", ".shadow" and ".sample" suffix.

".in" can always be applied, must be the first prefix.

opnsense/core,opnsense/core,opnsense/core,opnsense/core,opnsense/core,opnsense/core