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>

ipn/ipnlocal/web_client_stub.go

@@ -8,15 +8,13 @@ package ipnlocal
 import (
 	"errors"
 	"net"
-
-	"tailscale.com/client/local"
 )
 
 const webClientPort = 5252
 
 type webClient struct{}
 
-func (b *LocalBackend) ConfigureWebClient(lc *local.Client) {}
+func (b *LocalBackend) ConfigureWebClient(any) {}
 
 func (b *LocalBackend) webClientGetOrInit() error {
 	return errors.New("not implemented")