[release-branch.go1.15] 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.

Updates #46867
Fixes #47014

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>
(cherry picked from commit 835d86a17e)
Reviewed-on: https://go-review.googlesource.com/c/go/+/332330
Reviewed-by: Jay Conrod <jayconrod@google.com>
Run-TryBot: Carlos Amedee <carlos@golang.org>

src/cmd/go/internal/modcmd/vendor.go

@@ -10,6 +10,7 @@ import (
 	"io"
 	"io/ioutil"
 	"os"
+	"path"
 	"path/filepath"
 	"sort"
 	"strings"
@@ -192,7 +193,7 @@ func copyMetadata(modPath, pkg, dst, src string) {
 		if modPath == pkg {
 			break
 		}
-		pkg = filepath.Dir(pkg)
+		pkg = path.Dir(pkg)
 		dst = filepath.Dir(dst)
 		src = filepath.Dir(src)
 	}