nanobsd: Use the real path for NANO_PACKAGE_DIR

As users may have the Pkg directory as a symbolic link to the NanoBSD
"package dump directory". In commit 9af130ae8c03, cust_pkgng() was
greatly improved, however as a side effect of using a nullfs mount, the
directories and files must not be symlinks.

Fix it by using NANO_PACKAGE_DIR realpath().

PR: 269884
Reviewed by: imp
Approved by: emaste (mentor)
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D47532