createBundleHostCacheEntry(bundleHostPkgInfo) {
    return { [TARGET_INSTANCE]: null, realPath: bundleHostPkgInfo.realPath, addonInstanceCache: new Map() };
  }
Creates a cache entry for the bundleHostCache. Because we want to use the same sort of proxy
for both bundle hosts and for 'regular' addon instances (though their cache entries have
slightly different structures) we'll use the Symbol from getAddonProxy.

@method createBundleHostCacheEntry
@param {PackageInfo} bundleHostPkgInfo bundle host's pkgInfo.realPath
@return {Object} an object in the form of a bundle-host cache entry