Migrate to checker-sdk-go v1.3.0 with standalone build tag

The SDK split the HTTP server scaffolding into the new
checker-sdk-go/checker/server subpackage. Update main.go to import
server and call server.New, and isolate the interactive form code
behind the standalone build tag so plugin/builtin builds skip
net/http entirely.

checker/definition.go

@@ -38,7 +38,7 @@ import (
 var Version = "built-in"
 
 // Definition returns the CheckerDefinition for the ping checker.
-func Definition() *happydns.CheckerDefinition {
+func (p *pingProvider) Definition() *happydns.CheckerDefinition {
 	return &happydns.CheckerDefinition{
 		ID:      "ping",
 		Name:    "Ping (ICMP)",
@@ -89,9 +89,7 @@ func Definition() *happydns.CheckerDefinition {
 				},
 			},
 		},
-		Rules: []happydns.CheckRule{
-			Rule(),
-		},
+		Rules: Rules(),
 		Interval: &happydns.CheckIntervalSpec{
 			Min:     1 * time.Minute,
 			Max:     1 * time.Hour,