code: public void taintPackagesThatMissArtifacts() {
          for (Package pkg : prev.packages().values()) {
              for (File f : pkg.artifacts().values()) {
                  if (!f.exists()) {
                      // Hmm, the artifact on disk does not exist! Someone has removed it....
                      // Lets rebuild the package.
                      taintPackage(pkg.name(), ""+f+" is missing.");
                  }
              }
          }
      }
docs: If artifacts have gone missing, force a recompile of the packages
  they belong to.