code: String convertModularFileName(String filename) {
          int slash = filename.indexOf('/');
          return filename.substring(0, slash)
                 + "/"
                 + filename.substring(slash+1).replace('/', '.');
      }
docs: Replaces all but the first occurrence of '/' with '.'. Assumes
  that the name is in the format module/pkg/pkg/classname.class.
  That is, the name should contain at least one '/' character
  separating the module name from the package-class name.
  
  @param filename the input filename
  @return the modular classname