use Tilt mappings when sorting candidates for dynamic requests

We're using AssetPack on a project organized "by layer" versus "by type". This means we sometimes have two files with the same basename in a given directory (e.g., "todo.coffee" and "todo.scss").

AssetPack's selection of local files has a bug which prevents it from serving the right file, as it doesn't currently consider dynamic file extensions when sorting the matches found in `dyn_local_file_for`.

This pull request fixes that bug and adds new tests to cover this scenario.