[release-branch.go1.9] cmd/go: fix 'go get' compatibility for direct download of vgo-aware module
    
    CL 109340 added “minimal module-awareness for legacy operation.”
    One part of that is reinterpreting imports inside code trees with go.mod files
    as using semantic import versioning, and converting them back to
    legacy import paths by stripping the major version element
    (for example, interpreting import "x.com/foo/v2/bar" as import "x.com/foo/bar").
    This rewrite was not being applied during "go get", with the effect that once
    you had the target code downloaded already, everything was fine,
    but it didn't download and build successfully the first time.
    
    Fixes #25687.
    Cherry-pick fixes #25691.
    
    Change-Id: I3e122efdc8fd9a0a4e66f5aa3e6a99f90c7df779
    Reviewed-on: https://go-review.googlesource.com/115797
    Run-TryBot: Russ Cox <rsc@golang.org>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    Reviewed-by: Bryan C. Mills <bcmills@google.com>
    Reviewed-on: https://go-review.googlesource.com/116175