message: Use the span filename to look up breakpoints
  Summary: Previously we used the module name, but it's much easier to use the span filename, and works better as the module name can be anything.
diff: @@ -103,8 +103,9 @@ impl Backend {
  let fun = |span, ctx: &mut EvaluationContext| {
  let stop = {
  let breaks = breakpoints.lock().unwrap();
  + let span_loc = ctx.look_up_span(span);
  breaks
  - .get(ctx.current_module_name())
  + .get(span_loc.file.name())
  .map(|set| set.contains(&span))
  .unwrap_or_default()
  };
  