Allow a `FileCollection` visitor to skip visiting the output of artifact transforms, and use this when serializing file collections to the instant execution cache. This is only intended to be a basic step towards allow visitors better control over what to visit and in what form to visit. In this step, the API is specific to artifact transforms but will be made more general in later changes.
Run external transforms in parallel when necessary. We now check if the initial artifact is in the in-memory cache already, and if it isn't, we run the transform in parallel. This allows for more parallelism on e.g. buildSrc change of updating the Android plugin.