derp/derpserver: split off derp.Server out of derp into its own package

This exports a number of things from the derp (generic + client) package
to be used by the new derpserver package, as now used by cmd/derper.

And then enough other misc changes to lock in that cmd/tailscaled can
be configured to not bring in tailscale.com/client/local. (The webclient
in particular, even when disabled, was bringing it in, so that's now fixed)

Fixes #17257

Change-Id: I88b6c7958643fb54f386dd900bddf73d2d4d96d5
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscaled/webclient.go

@@ -0,0 +1,21 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !ts_omit_webclient
+
+package main
+
+import (
+	"tailscale.com/client/local"
+	"tailscale.com/ipn/ipnlocal"
+	"tailscale.com/paths"
+)
+
+func init() {
+	hookConfigureWebClient.Set(func(lb *ipnlocal.LocalBackend) {
+		lb.ConfigureWebClient(&local.Client{
+			Socket:        args.socketpath,
+			UseSocketOnly: args.socketpath != paths.DefaultTailscaledSocket(),
+		})
+	})
+}