feature/featuretags: make QR codes modular (#18358)

QR codes are used by `tailscale up --qr` to provide an easy way to
open a web-page without transcribing a difficult URI. However, there’s
no need for this feature if the client will never be called
interactively. So this PR adds the `ts_omit_qrcodes` build tag.

Updates #18182

Signed-off-by: Simon Law <sfllaw@tailscale.com>

cmd/tailscale/deps_test.go

@@ -0,0 +1,22 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package main
+
+import (
+	"testing"
+
+	"tailscale.com/tstest/deptest"
+)
+
+func TestOmitQRCodes(t *testing.T) {
+	const msg = "unexpected with ts_omit_qrcodes"
+	deptest.DepChecker{
+		GOOS:   "linux",
+		GOARCH: "amd64",
+		Tags:   "ts_omit_qrcodes",
+		BadDeps: map[string]string{
+			"github.com/skip2/go-qrcode": msg,
+		},
+	}.Check(t)
+}