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>

wgengine/magicsock/magicsock_test.go

@@ -111,7 +111,7 @@ 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 := derpserver.NewServer(key.NewNode(), logf)
+	d := derpserver.New(key.NewNode(), logf)
 
 	httpsrv := httptest.NewUnstartedServer(derpserver.Handler(d))
 	httpsrv.Config.ErrorLog = logger.StdLogger(logf)