wgengine/router{,/osrouter}: split OS router implementations into subpackage

So wgengine/router is just the docs + entrypoint + types, and then
underscore importing wgengine/router/osrouter registers the constructors
with the wgengine/router package.

Then tsnet can not pull those in.

Updates #17313

Change-Id: If313226f6987d709ea9193c8f16a909326ceefe7
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

feature/buildfeatures/feature_osrouter_disabled.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Code generated by gen.go; DO NOT EDIT.
+
+//go:build ts_omit_osrouter
+
+package buildfeatures
+
+// HasOSRouter is whether the binary was built with support for modular feature "Configure the operating system's network stack, IPs, and routing tables".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_osrouter" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasOSRouter = false

feature/buildfeatures/feature_osrouter_enabled.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Code generated by gen.go; DO NOT EDIT.
+
+//go:build !ts_omit_osrouter
+
+package buildfeatures
+
+// HasOSRouter is whether the binary was built with support for modular feature "Configure the operating system's network stack, IPs, and routing tables".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_osrouter" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasOSRouter = true

feature/condregister/maybe_osrouter.go

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

feature/featuretags/featuretags.go

@@ -126,6 +126,16 @@ var Features = map[FeatureTag]FeatureMeta{
 		Desc: "Outbound localhost HTTP/SOCK5 proxy support",
 		Deps: []FeatureTag{"netstack"},
 	},
+	"osrouter": {
+		Sym:  "OSRouter",
+		Desc: "Configure the operating system's network stack, IPs, and routing tables",
+		// TODO(bradfitz): if this is omitted, and netstack is too, then tailscaled needs
+		// external config to be useful. Some people may want that, and we should support it,
+		// but it's rare. Maybe there should be a way to declare here that this "Provides"
+		// another feature (and netstack can too), and then if those required features provided
+		// by some other feature are missing, then it's an error by default unless you accept
+		// that it's okay to proceed without that meta feature.
+	},
 	"portlist":   {"PortList", "Optionally advertise listening service ports", nil},
 	"portmapper": {"PortMapper", "NAT-PMP/PCP/UPnP port mapping support", nil},
 	"netlog": {