Prior to this it would download Go, setup the environment, then (maybe)
download Go again.
I renamed verify_go_version to be internal and fixed all callers.
The alternative to this would be to special-case code-generator. Since
it legit wants codegen, it seems wrong to make it be _examples (which tools
should ignore).
Make examples an "internal module" so the main go.mod for
k8s.io/code-generator does not get too polluted.
