Automatically generate @dir entries for package list if :debug is used.

Tested with a modified sysutils/biosfont in which the :debug option was added,
a file was installed into ${KERN_DEBUGDIR}/${KMODDIR} and that file was added
to pkg-plist.

PR: 216879 (inspired by)
Submitted by: Sergey Kozlov (kozlov.sergey.404@gmail.com)