proposal: module-aware gomodmerge

[myitcv]

Consider https://gist.github.com/myitcv/f7270ab81ab45aa286f496264f034b56

The use of gohack just to achieve a temporary go.mod that can then be used by gomodmerge is a bit verbose to say the least.

Instead, it might be better to have gomodmerge understand modules and hence be able to automate a lot of this (in a temporary directory).

Taking the linked example, this would transform:

$ gohack get -vcs github.com/docker/docker
$ docker=$(go list -m -f "{{.Dir}}" github.com/docker/docker)
$ pushd $docker
$ rm go.mod
$ go mod init
$ go mod tidy
$ popd
$ gohack undo github.com/docker/docker
$ gomodmerge $docker/go.mod

to simply:

$ gomodmerge github.com/docker/docker

where the version of github.com/docker/docker is resolved via the main module's go.mod in the "usual way".