This refactors DefaultPackage into types.Package and gets rid of the
interface. This is a preceeding for a follow up where accessors will be
removed from the code.
It also does several cleanup, so we get rid also of some unneeded
dependencies.
It holds necessary state plus additional information relative to the
context which we are being run to (e.g. if we are in a terminal or not).
Besides in the future we can use it also as a contextual logger to
provide more smart logging capabilities.
This also replace the general global configuration instance that
previously was share between the core components.
A new keyword `join` is introduced to generate the parent image. It
takes precedence over a `requires` or a `image` already defined in a
spec.
It will generate all the artifacts from the packages listed and join
them in a single image which will be used as parent for the package
build process.
This is a change which invalidates priorly generated hashes.
Fixes#173
