Get the Installer via DI

core/Command/App/Install.php

core/Command/Maintenance/Install.php

core/Command/Upgrade.php

core/ajax/update.php

core/register_command.php

lib/base.php

lib/private/Server.php

lib/private/Setup.php

lib/private/Updater.php

lib/private/legacy/app.php

lib/private/legacy/util.php

settings/ajax/updateapp.php

tests/lib/InstallerTest.php

tests/lib/SetupTest.php

tests/lib/UpdaterTest.php