Reuse downloaded packages instead of downloading again

[doom]

In some cases (e.g. re-installation), packages are downloaded again from their repository. It would be better to reuse downloaded packages (cached in <chroot>/var/nest/downloaded) if they are valid.

In order to be considered valid for reuse, the signature of the package will be calculated and compared to the signature obtained from the repository the package was downloaded from.
If the two signatures match, the downloaded package can be reused.