Add luet reclaim

Reclaim allows to migrate between different system layouts. This
is a experimental feature (yet) and might be revisited in the future.

This change:

- Adds Reclaim(system) to Installer
- Adds unit tests

Relates to #86

pkg/installer/interface.go

@@ -26,6 +26,8 @@ type Installer interface {
 	Install(pkg.Packages, *System) error
 	Uninstall(pkg.Package, *System) error
 	Upgrade(s *System) error
+	Reclaim(s *System) error
+
 	Repositories([]Repository)
 	SyncRepositories(bool) (Repositories, error)
 	Swap(pkg.Packages, pkg.Packages, *System) error