cmd/go: use path.Dir instead of filepath.Dir for package paths in 'go mod vendor'

copyMetadata walk-up to parent directory until the pkg become modPath.
But pkg should be slash-separated paths. It have to use path.Dir instead of
filepath.Dir.

Fixes #46867

Change-Id: I44cf1429fe52379a7415b94cc30ae3275cc430e8
Reviewed-on: https://go-review.googlesource.com/c/go/+/330149


Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Bryan C. Mills <bcmills@google.com>
Trust: Alexander Rakoczy <alex@golang.org>
Trust: Carlos Amedee <carlos@golang.org>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>