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.

DefaultURLRule is the default rule used by webpackager.Packager.

URLRule decides the validity URL of a resource.