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>

wgengine/userspace.go

@@ -51,7 +51,6 @@ import (
 	"tailscale.com/util/testenv"
 	"tailscale.com/util/usermetric"
 	"tailscale.com/version"
-	"tailscale.com/wgengine/capture"
 	"tailscale.com/wgengine/filter"
 	"tailscale.com/wgengine/magicsock"
 	"tailscale.com/wgengine/netlog"
@@ -1594,7 +1593,7 @@ var (
 	metricNumMinorChanges = clientmetric.NewCounter("wgengine_minor_changes")
 )
 
-func (e *userspaceEngine) InstallCaptureHook(cb capture.Callback) {
+func (e *userspaceEngine) InstallCaptureHook(cb packet.CaptureCallback) {
 	e.tundev.InstallCaptureHook(cb)
 	e.magicConn.InstallCaptureHook(cb)
 }