Mark coverage plugin as optional (#931)

* Mark coverage plugin as optional

* spotless fix

Author: vwagh-dev

pipeline-maven/pom.xml

@@ -52,6 +52,7 @@
     <dependency>
       <groupId>io.jenkins.plugins</groupId>
       <artifactId>coverage</artifactId>
+      <optional>true</optional>
     </dependency>
     <dependency>
       <groupId>org.jenkins-ci.main</groupId>

pipeline-maven/src/main/java/org/jenkinsci/plugins/pipeline/maven/publishers/CoveragePublisher.java

@@ -6,7 +6,6 @@
 
 import edu.umd.cs.findbugs.annotations.CheckForNull;
 import edu.umd.cs.findbugs.annotations.NonNull;
-import hudson.Extension;
 import hudson.FilePath;
 import hudson.model.TaskListener;
 import io.jenkins.plugins.coverage.metrics.steps.CoverageStep;
@@ -30,6 +29,7 @@
 import org.jenkinsci.plugins.pipeline.maven.MavenSpyLogProcessor;
 import org.jenkinsci.plugins.pipeline.maven.Messages;
 import org.jenkinsci.plugins.pipeline.maven.util.XmlUtils;
+import org.jenkinsci.plugins.variant.OptionalExtension;
 import org.jenkinsci.plugins.workflow.steps.StepContext;
 import org.jenkinsci.plugins.workflow.steps.StepExecution;
 import org.kohsuke.stapler.DataBoundConstructor;
@@ -246,7 +246,7 @@ private SourceCodeDirectory toSourceDirectory(Element e) {
     }
 
     @Symbol("coveragePublisher")
-    @Extension
+    @OptionalExtension(requirePlugins = "coverage")
     public static class DescriptorImpl extends AbstractHealthAwarePublisher.DescriptorImpl {
         @NonNull
         @Override