bsdinstall: allow an install script to access packages on the DVD

If installing from the DVD, mount its packages in the chroot at
/dist/packages. That way they'll be accessible to an install script.

MFC after: 2 weeks
Sponsored by: Axcient
Reviewed by: gjb
Differential Revision: https://reviews.freebsd.org/D35330

(cherry picked from commit 6a02539959cb36c4777f962d3d7ef4a76fbe0c24)