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/integration/integration.go

@@ -296,7 +296,7 @@ func exe() string {
 func RunDERPAndSTUN(t testing.TB, logf logger.Logf, ipAddress string) (derpMap *tailcfg.DERPMap) {
 	t.Helper()
 
-	d := derpserver.NewServer(key.NewNode(), logf)
+	d := derpserver.New(key.NewNode(), logf)
 
 	ln, err := net.Listen("tcp", net.JoinHostPort(ipAddress, "0"))
 	if err != nil {