packer-vmm - how to retrieve go dependencies?

Hi porters,

it's time to make a port out of my packer-vmm plugin.
https://github.com/double-p/packer-builder-openbsd-vmm

As this is my first ports try at all, I am missing something blatantly
obvious.

The Makefile is in 'examples/Makefile.ports' and trying a
'make fake' per Porters Guide, this fails like so:
===
==> Building for packer-builder-openbsd-vmm-0.9.0
cd
/usr/ports/pobj/packer-builder-openbsd-vmm-0.9.0/go/src/github.com/double-p/packer-builder-openbsd-vmm
&& GOMAXPROCS=1
GOCACHE=/usr/ports/pobj/packer-builder-openbsd-vmm-0.9.0/go-cache
GOPATH=/usr/ports/pobj/packer-builder-openbsd-vmm-0.9.0/go make
builder/openbsd-vmm/builder.go:9:2: cannot find package
"github.com/hashicorp/packer/common" in any of:
/usr/local/go/src/github.com/hashicorp/packer/common (from $GOROOT)
/usr/ports/pobj/packer-builder-openbsd-vmm-0.9.0/go/src/github.com/hashicorp/packer/common
(from $GOPATH)
===
..and more of the same.

Clearly I need to (let) fetch the required (sub)packages - but how? List
all subs in go.mod?
I was looking into several lang/go ports, but cant say I am getting the
magic sauce :)

TIA,
--
pb