From hashicorp/packer#2576 (comment)
cd ~/Source/go
$ENV:GOPATH=$(pwd)
go get github.com/mitchellh/packer
cd src\github.com\mitchellh\packer
git remote add hyperv https://github.com/patricklang/packer
git fetch hyperv HyperV
git checkout HyperV
go build -o bin/packer.exe .
Running it
& .\bin\packer.exe