So with the system of mirrors that is in place with distributing some open source software (e.g. debian, ubuntu, etc.) this is less true. A local mirror could selectively serve bad packages (and serve the correct packages to the verification bots).
Debian has a pretty nice mirroring system. Not only are all packages signed, but the Release file (which includes checksums of package lists) is also signed, preventing a mirror from omitting packages. For repositories which receive security updates (say, wheezy-updates), the index is valid for only few days in the future, which helps to prevent mirrors from withholding security updates [1].
If a mirror isn't updated, the user is eventually warned during updates:
It mostly negates the need for https mirrors for authenticity, although many still offer it. To my knowledge, most projects with mirror networks operate similar to this.
Actually, there's no requirement that .deb packages are signed. The system still provides a strong guarantee, because the releases file contains a list of checksums for each package, so it's impossible to tamper with the package, even though it's unsigned. However, if you manually download the package, all bets are off.
