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/derper/cert_test.go

@@ -22,8 +22,8 @@ import (
 	"testing"
 	"time"
 
-	"tailscale.com/derp"
 	"tailscale.com/derp/derphttp"
+	"tailscale.com/derp/derpserver"
 	"tailscale.com/net/netmon"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/key"
@@ -131,9 +131,9 @@ func TestPinnedCertRawIP(t *testing.T) {
 	}
 	defer ln.Close()
 
-	ds := derp.NewServer(key.NewNode(), t.Logf)
+	ds := derpserver.NewServer(key.NewNode(), t.Logf)
 
-	derpHandler := derphttp.Handler(ds)
+	derpHandler := derpserver.Handler(ds)
 	mux := http.NewServeMux()
 	mux.Handle("/derp", derpHandler)