cmd/go: add subdirectory support to go-import meta tag
This CL adds ability to specify a subdirectory in the go-import meta tag.
A go-import meta tag now will support:
<meta name="go-import" content="root-path vcs repo-url subdir">
Fixes: #34055
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-windows-amd64-longtest
Change-Id: Iedac520f97e0646254cc1bd2f97d5a9a5236829b
Reviewed-on: https://go-review.googlesource.com/c/go/+/625577 Reviewed-by: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Sam Thanawalla <samthanawalla@google.com>