internal/frontend: omit cmd packages from /std

Packages in cmd/ are now omitted from the directory listing on
pkg.go.dev/std, as in the case of golang.org/pkg.

For golang/go#44356

Change-Id: I160965d1b914a11863a815db3184d8b9823e8ae1
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/296951
Trust: Julie Qiu <[email protected]>
Reviewed-by: Jonathan Amsterdam <[email protected]>

Author: julieqiu

internal/frontend/directory.go

@@ -10,6 +10,7 @@ import (
 	"strings"
 
 	"golang.org/x/pkgsite/internal"
+	"golang.org/x/pkgsite/internal/stdlib"
 )
 
 // UnitDirectory is the union of nested modules and subdirectories for a
@@ -116,6 +117,12 @@ func getSubdirectories(um *internal.UnitMeta, pkgs []*internal.PackageMeta) []*S
 		if um.Path == pm.Path {
 			continue
 		}
+		if um.Path == stdlib.ModulePath && strings.HasPrefix(pm.Path, "cmd/") {
+			// Omit "cmd" from the directory listing on
+			// pkg.go.dev/std, since go list std does not
+			// list them.
+			continue
+		}
 		sdirs = append(sdirs, &Subdirectory{
 			URL:      constructUnitURL(pm.Path, um.ModulePath, linkVersion(um.Version, um.ModulePath)),
 			Suffix:   internal.Suffix(pm.Path, um.Path),