tool/gocross: break circular dependency on tailcfg (#15829)

Instead of using the version package (which depends on
tailcfg.CurrentCapabilityVersion) to get the git commit hash, do it
directly using debug.BuildInfo. This way, when changing struct fields in
tailcfg, we can successfully `go generate` it without compiler errors.

Updates #9634
Updates https://github.com/tailscale/corp/issues/26717

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

tool/gocross/gocross_test.go

@@ -0,0 +1,19 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package main
+
+import (
+	"testing"
+
+	"tailscale.com/tstest/deptest"
+)
+
+func TestDeps(t *testing.T) {
+	deptest.DepChecker{
+		BadDeps: map[string]string{
+			"tailscale.com/tailcfg": "circular dependency via go generate",
+			"tailscale.com/version": "circular dependency via go generate",
+		},
+	}.Check(t)
+}