Check if artifact exits before attach it to not get warned by maven

MavenProjectHelper claims it can replace artifacts but this generates a
warning for the user, to prevent this we now check for an already
attached artifact and simply update the file (even though it should
already point to the right one).

(cherry picked from commit 2c27d42)

tycho-p2-plugin/src/main/java/org/eclipse/tycho/plugins/p2/P2MetadataMojo.java

@@ -202,10 +202,10 @@ protected void attachP2Metadata() throws MojoExecutionException {
             File contentsXml = new File(targetDir, TychoConstants.FILE_NAME_P2_METADATA);
             File artifactsXml = new File(targetDir, TychoConstants.FILE_NAME_P2_ARTIFACTS);
             p2generator.persistMetadata(generatedMetadata, contentsXml, artifactsXml);
-            projectHelper.attachArtifact(project, TychoConstants.EXTENSION_P2_METADATA,
-                    TychoConstants.CLASSIFIER_P2_METADATA, contentsXml);
-            projectHelper.attachArtifact(project, TychoConstants.EXTENSION_P2_ARTIFACTS,
-                    TychoConstants.CLASSIFIER_P2_ARTIFACTS, artifactsXml);
+            attachArtifact(project, TychoConstants.EXTENSION_P2_METADATA, TychoConstants.CLASSIFIER_P2_METADATA,
+                    contentsXml);
+            attachArtifact(project, TychoConstants.EXTENSION_P2_ARTIFACTS, TychoConstants.CLASSIFIER_P2_ARTIFACTS,
+                    artifactsXml);
 
             ReactorProject reactorProject = DefaultReactorProject.adapt(project);
 
@@ -231,6 +231,21 @@ protected void attachP2Metadata() throws MojoExecutionException {
         writeArtifactLocations(localArtifactsFile, getAllProjectArtifacts(project));
     }
 
+    /**
+     * Performs an add or replace of the specified artifact, even though javadoc of
+     * {@link MavenProjectHelper} claims it can replace artifacts that generates a warning in recent
+     * maven versions.
+     */
+    private void attachArtifact(MavenProject project, String type, String classifier, File file) {
+        for (Artifact artifact : project.getAttachedArtifacts()) {
+            if (classifier.equals(artifact.getClassifier()) && type.equals(artifact.getType())) {
+                artifact.setFile(file);
+                return;
+            }
+        }
+        projectHelper.attachArtifact(project, type, classifier, file);
+    }
+
     private static boolean hasAttachedArtifact(MavenProject project, String classifier) {
         for (Artifact artifact : project.getAttachedArtifacts()) {
             if (classifier.equals(artifact.getClassifier())) {