tarball.LayerFromReader slurps the whole src in memory. The payoff is
that we might read the file multiple time as internally it's called
multiple times.
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.
