addEntry(entryName, entryVal) {
    this.hasEntries = true;
    this.entries[entryName] = entryVal;
  }
Add an entry (PackageInfo or NodeModulesList instance) to the entries
for this list. This is only called by PackageInfoCache. It is not intended
to be called directly by anything else.

@protected
@method addEntry
@param {String} entryName the name of the entry, i.e., the name of the
file or subdirectory in the directory listing.
@param {Object} entryVal the PackageInfo or NodeModulesList tha corresponds
to the given entry name in the file system.