Support #PREFETCH keyword

[yiranwang52]

Is your feature request related to a problem? Please describe.
In Java and NodeJS builds, a lot of time was spent on fetching dependencies. If dependencies from previous build are available on the filesystem, this part could be much faster.

Describe the solution you'd like
Support a #PREFETCH keyword, that fetches corresponding layer from previous build. This needs to be coupled with a cache service that can guess what the expected layer is.

Additional context
This should change the layer size; Optimization for layer size should be done by #PATCH keyword (see #257).