bsdinstall: Install pkg if it's available

Now that the pkg package is shipped on the pkgbase release media,
install it by default for a pkgbase install if it's present.

If it's not available (e.g., when running bsdinstall from a repository
built from src alone, without ports), skip it and assume the user will
install it another way.

MFC after: 1 day
Reviewed by: ifreund_freebsdfoundation.org, cperciva
Differential Revision: https://reviews.freebsd.org/D52639