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>

Author: mattn

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

@@ -13,6 +13,7 @@ import (
 	"io"
 	"io/fs"
 	"os"
+	"path"
 	"path/filepath"
 	"sort"
 	"strings"
@@ -299,7 +300,7 @@ func copyMetadata(modPath, pkg, dst, src string, copiedFiles map[string]bool) {
 		if modPath == pkg {
 			break
 		}
-		pkg = filepath.Dir(pkg)
+		pkg = path.Dir(pkg)
 		dst = filepath.Dir(dst)
 		src = filepath.Dir(src)
 	}