Package signing by the package's author /is/ the solution, right? This way the only entity that can publish a valid update is the the package author. This still has the problem of bootstrapping the trust relationship, and leaves the door open to the author publishing a malicious but signed package or the signing key being stolen and used to do the same. However if you don't trust the package author to be responsible or take precautions against the keys being stolen then you're essentially taking on the burden of implementing of your required functionality.