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>

wgengine/magicsock/magicsock_test.go

@@ -39,8 +39,7 @@ import (
 	"golang.org/x/net/ipv4"
 	"tailscale.com/cmd/testwrapper/flakytest"
 	"tailscale.com/control/controlknobs"
-	"tailscale.com/derp"
-	"tailscale.com/derp/derphttp"
+	"tailscale.com/derp/derpserver"
 	"tailscale.com/disco"
 	"tailscale.com/envknob"
 	"tailscale.com/health"
@@ -112,9 +111,9 @@ func (c *Conn) WaitReady(t testing.TB) {
 }
 
 func runDERPAndStun(t *testing.T, logf logger.Logf, l nettype.PacketListener, stunIP netip.Addr) (derpMap *tailcfg.DERPMap, cleanup func()) {
-	d := derp.NewServer(key.NewNode(), logf)
+	d := derpserver.NewServer(key.NewNode(), logf)
 
-	httpsrv := httptest.NewUnstartedServer(derphttp.Handler(d))
+	httpsrv := httptest.NewUnstartedServer(derpserver.Handler(d))
 	httpsrv.Config.ErrorLog = logger.StdLogger(logf)
 	httpsrv.Config.TLSNextProto = make(map[string]func(*http.Server, *tls.Conn, http.Handler))
 	httpsrv.StartTLS()