Function publish_plan 

pub fn publish_plan(
    previously_dirty: &HashSet<Url>,
    new_by_uri: HashMap<Url, Vec<Diagnostic>>,
) -> (Vec<(Url, Vec<Diagnostic>)>, HashSet<Url>)

Compute the publishes for one analysis round: every URI with new diagnostics, plus an empty publish for every URI that carried diagnostics last round and no longer does (the clear). Returns the publish list and the next round’s dirty set.