* Rename TransactionHandler -> InstallationInterface and
DaemonClientTransactionHandler -> ClientInstallationInterface.
* Add InstallationInterface::InitInstalledRepository(). Use in
_AddInstalledRepository() to get the repository packages instead of
using the package roster. The ClientInstallationInterface
implementation does it that way.
* Move RepositoryBuilder class to libpackage and add B* prefix to name.
* Pull BPackageManager class out of PackageManager and move to
libpackage. The base class is customizable via three handler objects
responsible for transaction handling, request execution, respectively
user interaction.
* Reorganize _ApplyPackageChanges(): Now we first prepare the
transactions for all affected installation locations (downloading
files etc.) and then commit them.
