Support reading json/yaml in build file

[raphaelvigee]

More generally, we would want to support reading in from any source (like kcl)

There are 2 ways to do this:

1. kcl is installed on the host, then heph would simply call into kcl when parsing build files
2. a set of "precursor" BUILD files are declared, allowing heph to run those first, install those tools, to be used by itself to run the rest of the BUILD files
3. A plugin system, would need some solid infrastructure to support (basically some kind of dependency management, would be useful for the roots too)