scripts: Upgrade process should utilize Go modules.

[Alan Noble]

The upgrade version can still be specified by a Git tag, however the tag will simply be used to write a go.mod file, and dependencies left up to the Go module system.

[kortschak]

Given the existence of module-aware go get now, I would suggest using go get package@version. This gets rid of the git convolution that currently exists and also stops us pulling irrelevant non-Go directories (if there are asset directories without Go source code in them, those would need to be fixed), saving bandwidth.

It also gets rid of the existence of the make calls, which from my just not survey are not needed at all.

[Alan Noble]

Yes, I agree there is no need to use Git (at least for this). In fact, no need to explicitly call go get either. That can be left to the Go module system.

Conceptually the new upgrade process is:

1. Write go.mod
2. Call go build

[kortschak]

That's more complicated than it needs to be; just calling go get does that without having to write to go.mod and automatically invokes go build.