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.