- These don't have to be static methods of PackageItem, a free
function is more convenient.
- Since it's not API of PackageItem anymore, need to
- update tests not to use API
- do API-not-available warnings in consumers
- Put the implementation entirely in a separate file, keep the
not-supported one in PackageModel.cpp (but only in an #ifdef).
- Makes the various optional-data-sources more similar.