Function pack_from_sources 

pub fn pack_from_sources(
    info: Source<'_>,
    ui: Source<'_>,
    english: &BTreeMap<String, String>,
    rules: &PackRules<'_>,
) -> Result<LangPack, Vec<ContentError>>

Parses and validates a pack with screen text only: its lang.ron (info) and its ui.ron (ui), checked against english. See pack_from_files.