Add functional tests for "kpm pack"

Please help to review functional tests for `kpm pack` @Eilon @davidfowl @lodejard @Praburaj.

I made utils that can create fake projects (directory tree) based on a directory structure represented by JSON. We can also represent an expected output directory structure with JSON and verify whether the actual output matches the expected output.
Can we add a helper on top of this one that knows how to properly execute kpm pack both on *nix and on windows?