bin/151187: pkg_add(1): allow different pkgdep location

[grarpamp]

One of the reasons this is needed is to allow
develop[ment/ers] of local code or packages to
easily bring in libraries and things they need.
Without having to build and deal with them
locally. Automation of build environments.

Less or selected bandwidth use.

Another case is openoffice. 2.4.x is outdated
and good-day does not carry the deps for any
version, thus it needs manual intervention and
pointing to freebsd.org/mirror to install the deps.