createAddonCacheEntry(addonInstance, addonRealPath) {
    return { [TARGET_INSTANCE]: addonInstance, realPath: addonRealPath };
  }
Create a cache entry object for a given (non-bundle-host) addon to put into
an addon cache.

@method createAddonCacheEntry
@param {Addon} addonInstance the addon instance to cache
@param {String} addonRealPath the addon's pkgInfo.realPath
@return {Object} an object in the form of an addon-cache entry