Function platform_feature_sets

pub fn platform_feature_sets<'a>(
    package: &'a PackageMetadata<'a>,
) -> BTreeMap<PlatformVariant, FeatureSet<'_>>

Returns a FeatureSet of reverse dependencies (in other words, all crates that depend on) for the provided package, for every platform that we support.

TODO(parkmycar): Make the list of platforms configurable.