Improve Go module support in Makefile

[lblackstone]

No description provided.

[hausdorff]

This seems reasonable but for my own education can you explain the changes in Makefile? I am woefully uneducated. :(

[lblackstone]

@hausdorff The GO111MODULE=on statement makes these commands work whether or not they are executed within a GOPATH. The GOPROXY statement downloads deps from a proxy rather than hitting all of the individual repos, and go mod tidy updates the dependency list and pulls modules as necessary.

[hausdorff]

I believe go mod tidy does not actually work in GOPATH? At least, not on my machine. In GOPATH I find I have to run go mod vendor

[lblackstone]

@hausdorff Right, it won't work unless you set the GO111MODULE=on env var. It will work either way with that.