Check whether or not integrations stage has been restarted (elastic#4748)

Add a check before running the "Check integrations" stage to ensure
that exists the packages folder. This stage requires to be checkout the
repository and this is performed in the first stage.

Author: mrodm

.ci/Jenkinsfile

@@ -84,6 +84,9 @@ pipeline {
     stage('Check integrations') {
       steps {
         script {
+          if (!fileExists("${BASE_DIR}/packages")) {
+            error "Has the stage been restarted? This pipeline requires to be restarted running all stages."
+          }
           dir("${BASE_DIR}/packages") {
             def integrations = [:]
             // Include hack to skip temporary files with "@tmp" suffix.