Use go modules for dependency tracking and fix broken dependency (go-env)

[zph]

This pr introduces use of go mod for handling dependencies.

It also removes go-env, used in 1 location but broken in newer versions. I replaced it with stdlib code. Fixes warnings/error from mmake.go.

[zph]

(I think CI needs updating/debugging. make test and go build cmd/mmake/mmake.go are working locally)

[zph]

@tj I don't see a mechanism for me to update CI based on my Github contributor role. But I think it needs a little updating.

If you want to add me there, I can get it working again. I'd also be happy to set it up on Travis or Circle, both of which I'm familiar with.

[zph]

I believe the three currently correct steps in Semaphore, as of this PR are:

* `go mod download`
* `go build -v ./...`
* `go test -v ./...`

This step go get github.com/campoy/tools/tree can be dropped thanks to go mod handling those duties.

[tj]

hmm Semaphore seems messed, not getting any output, we could probably just use GH actions — I'll remove Semaphore for now :D