Currently we can record a dependency on external file using `addDependentFile`. But we cannot record dependency on external directory. Some code traverses directories, and recompilation should be triggered if *contents* of the directory change, especially if new files are added. If directories were just files a la Plan9, `addDependentFile` would be enough, but this is not the case on current operating systems. Note: `addDependentDirectory` only needs to be shallow, i.e. record the contents of the directory (e.g. hash of `listDirectory`). Deep dependency can be recorded by `addDependentFile` (and `addDependentDirectory`) recursively.