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
Add api call which uses go-container registry to create OCI images from
standard tar archives.
Consume new API when generating final images instead of docker building them
and adapts/add tests as necessary.
This change now allows to carry over xattrs to final images.
Fixes#266
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.
Instead of merely storing files into disk use a real cache.
This also makes possible finally to reference artifacts in the cache with the
package checksum, which solves the cache hit checksum failures we had
previously.
