Prepares datasources in an MML file for consumption in Mapnik
As of carto 0.2.0, the Carto module expects all datasources and resources to be localized - remote references like URLs and providers are not downloaded when maps are rendered.
Millstone now contains this logic - it provides two functions,
flush. Resolve takes an MML file and returns a localized
flush can be used to clear the cache of a specific resource.