code: public static File getManifestFile(MavenProject proj, String pluginArtifactId) {
          Xpp3Dom dom = proj.getGoalConfiguration("org.apache.maven.plugins", pluginArtifactId, null, null);
          if (dom != null) {
              Xpp3Dom archive = dom.getChild("archive");
              if (archive != null) {
                  Xpp3Dom val = archive.getChild("manifestFile");
                  if (val != null) {
                      return new File(proj.getBasedir().getAbsolutePath() + "/" + val.getValue());
                  }
              }
          }
          return null;
      }
docs: Get manifest file from plugin configuration
  @param proj
  @param pluginArtifactId
  @return the manifest file