cmd/go: allow verifying vendored code

[krancour]

go mod verify is extremely useful for validating the integrity of modules in the local cache.

It would be great if projects that choose to vendor their modules (then presumably building with go build -mod vendor ...) had a similar command to verify the integrity of modules in that directory.

This would satisfy a major requirement that many projects need to account for in their CI process-- ensuring that vendored code hasn't been tampered with.

[bcmills]

This is still on our radar, but probably not happening for 1.13. (We have a lot to do this cycle!)

I'm hoping to get to it in 1.14, but we don't have a 1.14 milestone defined yet.