Add support for building composite images

[dmikusa]

Describe the Enhancement

The tool should allow building a composite image.

Possible Solution

Initially, the tool is going to allow building from existing images, so if you wanted to build a custom buildpack and include that in a composite image, you'd need to first run the tool to build the component buildpack and then run it to build the composite buildpack.

This will likely be a thin wrapper over pack.

Motivation

Make it easier to build composites. Start simple. We're not supporting build from a local directory, at least not initially. We'll add that in as a clear use case and requirements develop.