Fix subpackage import vulnerability

[aymeric-roucher]

A malicious LLM could execute malicious code by using subpackages from authorized packages, like this exploiting the fact that random is authorized:

import random
random._os_system("malicious bash command")

It required a supply chain attack to have user run a malicious LLM, but it could work.
So this PR fixes it!

[albertvillanova]

Thanks for the security fix.

[stackviolator]

sorry for long wait.. post finally got cleared! wrote up a blog about the technical details on the jailbreak + malicious import. thank you guys for the cooperation and super quick fix :)

https://x.com/joshtmerrill/status/1890483661151056261