feature/capture: move packet capture to feature/*, out of iOS + CLI

We had the debug packet capture code + Lua dissector in the CLI + the
iOS app. Now we don't, with tests to lock it in.

As a bonus, tailscale.com/net/packet and tailscale.com/net/flowtrack
no longer appear in the CLI's binary either.

A new build tag ts_omit_capture disables the packet capture code and
was added to build_dist.sh's --extra-small mode.

Updates #12614

Change-Id: I79b0628c0d59911bd4d510c732284d97b0160f10
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

feature/condregister/maybe_capture.go

@@ -0,0 +1,8 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !ios && !ts_omit_capture
+
+package condregister
+
+import _ "tailscale.com/feature/capture"