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
