tos3to4: flush localrepo on migration

This flushes localrepo store on migration. We have to do that to ensure stability of system. Users can and do have binary packages there that are going to be invalid with new libc. Some of them are important packages and are possibly causing stability issues.

Users still can access their old packages to recover them in updater's standard directory for collided files.

We notify user about that using notification system.

This reuses script that was changing index format of localrepo.

Implements point in https://gitlab.labs.nic.cz/turris/turris-os-packages/issues/344