cmd/tailscaled: support "ts_omit_ssh" build tag to remove SSH

Some environments would like to remove Tailscale SSH support for the
binary for various reasons when not needed (either for peace of mind,
or the ~1MB of binary space savings).

Updates tailscale/corp#24454
Updates #1278
Updates #12614

Change-Id: Iadd6c5a393992c254b5dc9aa9a526916f96fd07a
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscaled/deps_test.go

@@ -0,0 +1,30 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package main
+
+import (
+	"testing"
+
+	"tailscale.com/tstest/deptest"
+)
+
+func TestOmitSSH(t *testing.T) {
+	const msg = "unexpected with ts_omit_ssh"
+	deptest.DepChecker{
+		GOOS:   "linux",
+		GOARCH: "amd64",
+		Tags:   "ts_omit_ssh",
+		BadDeps: map[string]string{
+			"tailscale.com/ssh/tailssh":            msg,
+			"golang.org/x/crypto/ssh":              msg,
+			"tailscale.com/sessionrecording":       msg,
+			"github.com/anmitsu/go-shlex":          msg,
+			"github.com/creack/pty":                msg,
+			"github.com/kr/fs":                     msg,
+			"github.com/pkg/sftp":                  msg,
+			"github.com/u-root/u-root/pkg/termios": msg,
+			"tempfork/gliderlabs/ssh":              msg,
+		},
+	}.Check(t)
+}