pkgbase: do not record dependency on non-existent liby package

liby-dev provides (only) liby.a. liby has no headers or man pages, and
there is no liby package. Add a special case to record no dependency on
the package that does not exist.

PR: 266923
Reviewed by: bapt
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37429