derp/derpserver: clean up extraction of derp.Server (#17264)

PR #17258 extracted `derp.Server` into `derp/derpserver.Server`.

This followup patch adds the following cleanups:
1. Rename `derp_server*.go` files to `derpserver*.go` to match
   the package name.
2. Rename the `derpserver.NewServer` constructor to `derpserver.New`
   to reduce stuttering.
3. Remove the unnecessary `derpserver.Conn` type alias.

Updates #17257
Updates #cleanup

Signed-off-by: Simon Law <sfllaw@tailscale.com>

tstest/natlab/vnet/vnet.go

@@ -611,7 +611,7 @@ func newDERPServer() *derpServer {
 	ts.Close()
 
 	ds := &derpServer{
-		srv:       derpserver.NewServer(key.NewNode(), logger.Discard),
+		srv:       derpserver.New(key.NewNode(), logger.Discard),
 		tlsConfig: ts.TLS, // self-signed; test client configure to not check
 	}
 	var mux http.ServeMux