branch3: use an explicitely experimental name for files
Since this format is still experimental, we don't want to have to side-step
the `branch3` name in case people do start using it before it's stable.
This directory is meant to cache artifacts useful for tests (such as bundle).
Those artifacts need to be cached because they are slow to regenerate on each
test but too large to be tracked within the repository. They are not expected
to change between each run and can be cached.
The `./scripts/` contains code to generate the artifact while the `cache`
directory contains resulting artifact.