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/wgengine.go

@@ -11,10 +11,10 @@ import (
 
 	"tailscale.com/ipn/ipnstate"
 	"tailscale.com/net/dns"
+	"tailscale.com/net/packet"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/key"
 	"tailscale.com/types/netmap"
-	"tailscale.com/wgengine/capture"
 	"tailscale.com/wgengine/filter"
 	"tailscale.com/wgengine/router"
 	"tailscale.com/wgengine/wgcfg"
@@ -129,5 +129,5 @@ type Engine interface {
 	// InstallCaptureHook registers a function to be called to capture
 	// packets traversing the data path. The hook can be uninstalled by
 	// calling this function with a nil value.
-	InstallCaptureHook(capture.Callback)
+	InstallCaptureHook(packet.CaptureCallback)
 }