devel/dep: create port

Dep is a Go tool for managing vendor dependencies (not to be confused
with godep, a different, deprecated tool). dep updates the vendor/
directory contents based on a manifest, prunes unnecessary dependencies,
and adds missing dependencies to vendor/ as necessary.

dep is the heir-apparent for vendor management and is being actively
driven by the Go core team with an eye toward inclusion in the Go tool.

WWW: https://github.com/golang/dep

Submitted by: Sam Gwydir <sam.gwydir@joyent.com>
Reviewed by: tobik
Differential Revision: https://reviews.freebsd.org/D13364