freebsd-update: Use BASEDIR when checking for src component

src could potentially be installed under the based dir
and not under the root or vice versa.

PR: 224048
Submitted by: Gerald Aryeetey <aryeeteygerald_rogers.com>
Reviewed by: delphij
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D18849