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.
This commit is multi-fold as it also refactors internally context and logger
as interfaces so it is easier to plug luet as a library externally.
Introduces a garbage collector (related to #227) but doesn't handle yet
parallelism.
Closes#265
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.
