tstest/deptest: add test-only package to unify negative dep tests

Updates #8658

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tstest/deptest/deptest.go

@@ -0,0 +1,56 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// The deptest package contains a shared implementation of negative
+// dependency tests for other packages, making sure we don't start
+// depending on certain packages.
+package deptest
+
+import (
+	"encoding/json"
+	"os"
+	"os/exec"
+	"runtime"
+	"testing"
+)
+
+type DepChecker struct {
+	GOOS    string
+	GOARCH  string
+	BadDeps map[string]string // package => why
+}
+
+func (c DepChecker) Check(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		// Slow and avoid caring about "go.exe" etc.
+		t.Skip("skipping dep tests on windows hosts")
+	}
+	t.Helper()
+	cmd := exec.Command("go", "list", "-json", ".")
+	var extraEnv []string
+	if c.GOOS != "" {
+		extraEnv = append(extraEnv, "GOOS="+c.GOOS)
+	}
+	if c.GOARCH != "" {
+		extraEnv = append(extraEnv, "GOARCH="+c.GOARCH)
+	}
+	cmd.Env = append(os.Environ(), extraEnv...)
+	out, err := cmd.Output()
+	if err != nil {
+		t.Fatal(err)
+	}
+	var res struct {
+		Deps []string
+	}
+	if err := json.Unmarshal(out, &res); err != nil {
+		t.Fatal(err)
+	}
+
+	for _, dep := range res.Deps {
+		if why, ok := c.BadDeps[dep]; ok {
+			t.Errorf("package %q is not allowed as a dependency (env: %q); reason: %s", dep, extraEnv, why)
+		}
+	}
+	t.Logf("got %d dependencies", len(res.Deps))
+
+}