Build path and name mappings
Remove logging within matrix package
AssetDir instances know their root dir
Evaluate directives
Refactor common attributes of AssetFile and AssetDir into an interface
Refactor mappings to avoid name conflicts

(e.g. name mappings for file.js and file.css no longer conflict)
Add asset path in directive errors
Add some comments explaining the AssetPointer interface
Drop the `Asset` prefix
Rename `InputManifest` to `Manifest`

The name implied there is also an `OutputManifest` which there is not.