cmd/govulncheck: set gotypesalias=1 when using >=1.23 toolchain

Set gotypesalias=1 when using >=1.23 toolchain on all of
the main packages in x/tools that use go/types.

This effectively upgrades commit https://go.dev/cl/617095.

For golang/go#69772

Change-Id: I9f3e64d348f6bffc75321a08145fde07fb4024a6
Reviewed-on: https://go-review.googlesource.com/c/vuln/+/627715
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Zvonimir Pavlinovic <zpavlinovic@google.com>

cmd/govulncheck/gotypesalias.go

@@ -0,0 +1,12 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build go1.23
+
+//go:debug gotypesalias=1
+
+package main
+
+// Materialize aliases whenever the go toolchain version is after 1.23 (#69772).
+// Remove this file after go.mod >= 1.23 (which implies gotypesalias=1).