# Functions
AppendExtDotExchangeDate is like AppendExtDotLastModified but always uses vp.Date instead of the last modified time.
AppendExtDotLastModified generates the validity URL by appending ext and the resource's last modified time.
FixedURL uses url as the validity URL for all resources.
# Variables
DefaultURLRule is the default rule used by webpackager.Packager.
# Interfaces
URLRule decides the validity URL of a resource.