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>

feature/condregister/condregister.go

@@ -5,3 +5,10 @@
 // by build tags. It is one central package that callers can empty import
 // to ensure all conditional features are registered.
 package condregister
+
+// Portmapper is special in that the CLI also needs to link it in,
+// so it's pulled out into its own package, rather than using a maybe_*.go
+// file in condregister.
+import (
+	_ "tailscale.com/feature/condregister/portmapper"
+)

feature/condregister/maybe_debugportmapper.go

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

feature/condregister/portmapper/doc.go

@@ -0,0 +1,6 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package portmapper registers support for portmapper
+// if it's not disabled via the ts_omit_portmapper build tag.
+package portmapper

feature/condregister/portmapper/maybe_portmapper.go

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