cmd/go: add (and default to) `-buildvcs=auto` [1.18 backport]

[gopherbot]

@bcmills requested issue #51748 to be considered for backport to the next 1.18 minor release.

@gopherbot, please backport to Go 1.18. This will fix a usability regression for go build.

[dmitshur]

Now that this is approved and upstream issue is fixed, I took a look at creating a backport CL, and found there are a couple of conflicts involving load.PackageOpts.LoadVCS and Package.IsTestOnly. @bcmills Would you like to create the backport CL and resolve those conflicts? Thanks.

[dmitshur]

Whoops, the release-branch.go1.18 branch I tested with was out of date—it's just the trivial alldocs.go conflict, so I'll make the backport CL.

[gopherbot]

Change https://go.dev/cl/400454 mentions this issue: [release-branch.go1.18] cmd/go: allow '-buildvcs=auto' and treat it as the default