cmd/go: improve support for using modules in offline mode

[mvdan]

I was on a plane earlier this week, and not being able to use go get was unfortunate. Any queries like go get some-dependency@latest or @master always reach for the internet, failing immediately. Something akin to go get some-dependency@latest-cached would have been neat, because that dependency was many months out of date, and my cache had multiple newer tagged versions.

I ended up looking directly into my module download cache, finding the latest tagged version there, then using go get [email protected]. Which worked, but it's also somewhat tedious, and I don't like having to peek at the local cache in this way to be able to use it.

I believe that we've talked about better offline support before, but I could not find a tracking issue for it.

cc @bcmills @matloob

[bcmills]

Aha! I finally found the issue I had half-remembered for this.

[bcmills]

Duplicate of #43646

[mvdan]

Ah, thanks!