feature/portmapper: make the portmapper & its debugging tools modular

Starting at a minimal binary and adding one feature back...
    tailscaled tailscale combined (linux/amd64)
     30073135  17451704  31543692 omitting everything
    +  480302 +   10258 +  493896 .. add debugportmapper
    +  475317 +  151943 +  467660 .. add portmapper
    +  500086 +  162873 +  510511 .. add portmapper+debugportmapper

Fixes #17148

Change-Id: I90bd0e9d1bd8cbe64fa2e885e9afef8fb5ee74b1
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/netcheck/netcheck.go

@@ -33,7 +33,7 @@ import (
 	"tailscale.com/net/netmon"
 	"tailscale.com/net/netns"
 	"tailscale.com/net/ping"
-	"tailscale.com/net/portmapper"
+	"tailscale.com/net/portmapper/portmappertype"
 	"tailscale.com/net/sockstats"
 	"tailscale.com/net/stun"
 	"tailscale.com/syncs"
@@ -215,7 +215,7 @@ type Client struct {
 
 	// PortMapper, if non-nil, is used for portmap queries.
 	// If nil, portmap discovery is not done.
-	PortMapper *portmapper.Client // lazily initialized on first use
+	PortMapper portmappertype.Client
 
 	// UseDNSCache controls whether this client should use a
 	// *dnscache.Resolver to resolve DERP hostnames, when no IP address is
@@ -730,7 +730,7 @@ func (rs *reportState) probePortMapServices() {
 
 	res, err := rs.c.PortMapper.Probe(context.Background())
 	if err != nil {
-		if !errors.Is(err, portmapper.ErrGatewayRange) {
+		if !errors.Is(err, portmappertype.ErrGatewayRange) {
 			// "skipping portmap; gateway range likely lacks support"
 			// is not very useful, and too spammy on cloud systems.
 			// If there are other errors, we want to log those.