Refactor package structure and update refs

Author: Travis Clarke

prow/cmd/mkbuild-cluster/BUILD.bazel

@@ -6,7 +6,7 @@ go_library(
     importpath = "k8s.io/test-infra/prow/cmd/mkbuild-cluster",
     visibility = ["//visibility:private"],
     deps = [
-        "//pkg/gencred:go_default_library",
+        "//gencred/pkg/certificate:go_default_library",
         "//prow/kube:go_default_library",
         "@com_github_sirupsen_logrus//:go_default_library",
         "@io_k8s_api//core/v1:go_default_library",

prow/cmd/mkbuild-cluster/main.go

@@ -25,7 +25,7 @@ import (
 	"os/exec"
 	"strings"
 
-	"k8s.io/test-infra/pkg/gencred"
+	"k8s.io/test-infra/gencred/pkg/certificate"
 	"k8s.io/test-infra/prow/kube"
 
 	"github.com/sirupsen/logrus"
@@ -234,7 +234,7 @@ func describeCluster(o options, kubeContext string) (*describe, error) {
 		if err != nil {
 			return nil, fmt.Errorf("create client: %v", err)
 		}
-		certPEM, keyPEM, caPEM, err := gencred.CreateClusterCertificateCredentials(clientset)
+		certPEM, keyPEM, caPEM, err := certificate.CreateClusterCertificateCredentials(clientset)
 		if err != nil {
 			return nil, fmt.Errorf("create cert and key: %v", err)
 		}