This is step one towards supporting GOPATH natively. With this, one can
set GOPATH={work_dir} and go will find src that we've already checked
out, and will put artifacts in a sibling dir to src that is in the
GOPATH. We'll follow this up with putting the repos into
{src_dir}/{connection}/{full_repo} - such as
"git.openstack.org/openstack-infra/zuul"
Change-Id: I5acefd212587d18d0d3cc4ccd436555734e56e63
d642d8547c