DUB API breaking change: `Package.metadataCache` setter and getter have been removed

Those two functions were used to provide access to the metadata cache file
to the generator. They were never intended for public consumption,
and the JSON file format was not stable.

Due to the introduction of the build cache, they needed to be removed,
as there was no way to provide a sensible transition path, and they should be unused.
If you have a use case for it, please open an issue in dub repository.