Add support for Go 1.16

[rafikdraoui]

Go 1.16 changes the way modules are handled. It now expects Go projects to have non-empty go.mod files.

This change is compatible with Go 1.15.

Fixes #1815

---

The golang tests pass under all combinations of Go [1.15|1.16] + GO111MODULE=[""|"on"|"auto"].

I could also install and run the following hook successfully with both Go 1.15 and Go 1.16:

- repo: local
    hooks:
      - id: shfmt
        name: Format shell files with shfmt
        language: golang
        additional_dependencies: ["mvdan.cc/sh/v3/cmd/shfmt@v3.2.1"]
        types: [shell]
        entry: shfmt -w

These changes work for my use cases, and they seem sensible to me, but I'm not all that familiar with Go modules, so perhaps this has some implications I don't know about.

[asottile]

validated that this is the only reasonable fix -- even setting the remote properly does not convince go that it should build