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>

cmd/derper/cert_test.go

@@ -131,7 +131,7 @@ func TestPinnedCertRawIP(t *testing.T) {
 	}
 	defer ln.Close()
 
-	ds := derpserver.NewServer(key.NewNode(), t.Logf)
+	ds := derpserver.New(key.NewNode(), t.Logf)
 
 	derpHandler := derpserver.Handler(ds)
 	mux := http.NewServeMux()

cmd/derper/derper.go

@@ -188,7 +188,7 @@ func main() {
 
 	serveTLS := tsweb.IsProd443(*addr) || *certMode == "manual"
 
-	s := derpserver.NewServer(cfg.PrivateKey, log.Printf)
+	s := derpserver.New(cfg.PrivateKey, log.Printf)
 	s.SetVerifyClient(*verifyClients)
 	s.SetTailscaledSocketPath(*socket)
 	s.SetVerifyClientURL(*verifyClientURL)