Refactor reusable pkgman code into libpackage

* 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.

src/bin/pkgman/command_update.cpp

@@ -89,10 +89,7 @@ UpdateCommand::Execute(int argc, const char* const* argv)
 	const char* const* packages = argv + optind;
 
 	// perform the update
-	PackageManager packageManager(location,
-		PackageManager::ADD_INSTALLED_REPOSITORIES
-			| PackageManager::ADD_REMOTE_REPOSITORIES
-			| PackageManager::REFRESH_REPOSITORIES);
+	PackageManager packageManager(location);
 	packageManager.Update(packages, packageCount);
 
 	return 0;