Allow codegen for external packages (#107)
Allow codegen for external packages (#107)

External files have a short_path that looks like "../package/path" which
matches the path under `runfiles` but not under `execroot`.

The `path` property works better for external files, however it does not
work for generated local files. Neither `path` nor `short_path` work in
the case of external generated files.
dart-archive/rules_dart,dart-archive/rules_dart,dart-archive/rules_dart