Problem: a bare call in a Dart file resolved against every imported
library. An `import 'a.dart' as p;` still bound `foo()` to a.dart's foo,
`hide` and `show` combinators were ignored, and a name re-exported through
a barrel library (`export 'impl.dart';`) never resolved.
Fix: record each Dart import/export directive with its `as` prefix and
show/hide lists. A bare name resolves only in libraries imported without a
prefix whose combinators let it through, plus that library's own exports
that also let it through (one level).
Test: tests/dart_imports.rs covers a prefixed, a hiding, a showing and a
barrelled import; it failed on the prefixed case before this change.
