cmd/go: fix version stamping for v2 modules and subdirectories
We were not passing the module path to newCodeRepo which caused it to
incorrectly parse the major version. This allowed v0 and v1 modules to
work because an empty major version is allowed in that case.
Additionally we need to pass the root module path to derive the correct tag
for subdirectories.
Fixes: #72877 Fixes: #71738
Change-Id: Id792923f426858513972e713623270edbc76c545
Reviewed-on: https://go-review.googlesource.com/c/go/+/661875
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Michael Matloob <matloob@golang.org>