* Print all not-found packages
When trying to install several packages that are not found luet will now
print all packages that are not found, instead of only first one.
* changes to some failing tests
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