Fix consistency of output paths of assets

[matt-gadd]

Bug
We currently are a bit inconsistent in terms of where we place and how we name outputted assets.

For instance, for assets such as images, we always output them to the root of the output folder and they are always content hashed. For css and js, the assets are output to the directory structure they would've existed in the source of the project, and they are only hashed for a dist build.

We should normalize this by fixing one or the other.