Function flatten_source 

pub fn flatten_source(source: &str, source_dir: &Path) -> LingResult<Program>

Flatten a concatenated source string against source_dir, resolving its use imports and deduplicating definitions. A host that pastes several files together (e.g. the game launcher) and whose entry also uses some of them would otherwise produce duplicate top-level functions; the JIT requires each symbol once, so the last definition of each name wins (matching the tree-walker, which registers into a map).