code: public static Package getAssetPackage(Long assetId) throws CachingException {
          for (Package pkg : getPackageList()) {
              if (pkg.containsAsset(assetId))
                  return pkg;
          }
          return null;
      }
docs: Returns the design-time package for a specified assetId
  (Returns the first match so does not support the same asset in multiple packages).