golang-1.11-go and go.mod

["Jeremy C. Reed" <reed@fsi.io>]

I have some code that we switched to using golang-1.11-go using stretch 
backports (previously was version 1.7 or golang-go on stretch). My 
debian/rules has
# set executable path to see the golang-1.11-go install
export PATH := /usr/lib/go-1.11/bin:/usr/bin:/bin

The new code also introduced a go.mod file.

Now package building (dh_auto_build -O--buildsystem=golang) fails 
with:

exec: "git": executable file not found in $PATH

It is trying to fetch code instead of using the dev packages it already 
extracted to /usr/share/gocode/src ...

I changed debian/rules to remove the go.mod and then the package build 
is successful.

Also the switch to newer golang introduce another problem:

go: disabling cache (/nonexistent/.cache/go-build) due to initialization 
failure: mkdir /nonexistent: permission denied
go: cannot use modules with build cache disabled
dh_auto_build: go install -v -p 1 returned exit code 1

I worked around that in debian/rules with
export GOCACHE = /tmp/gocache
And target does mkdir -p /tmp/gocache

Can anyone point me to debian packaging guidelines for packaging with 
golang-1.11-go including with using go.mod?

Is removing go.mod or otherwise telling the build to not use it 
acceptable workaround?

Thanks,

Jeremy C. Reed