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>

net/captivedetection/captivedetection_test.go

@@ -15,7 +15,7 @@ import (
 	"testing"
 	"time"
 
-	"tailscale.com/derp/derphttp"
+	"tailscale.com/derp/derpserver"
 	"tailscale.com/net/netmon"
 	"tailscale.com/syncs"
 	"tailscale.com/tstest/nettest"
@@ -136,7 +136,7 @@ func TestAgainstDERPHandler(t *testing.T) {
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
 
-	s := httptest.NewServer(http.HandlerFunc(derphttp.ServeNoContent))
+	s := httptest.NewServer(http.HandlerFunc(derpserver.ServeNoContent))
 	defer s.Close()
 	e := Endpoint{
 		URL:                        must.Get(url.Parse(s.URL + "/generate_204")),