Another fun fact to consider is that with many package formats, you can execute arbitrary code at install time so if a malicious package can get into a repository, it's very likely to start compromising systems quickly.
Whilst a package manager repo. compromise would be the biggest bang in terms of attack, compromising the credentials of the developers of popualar libraries would be an easier attack (and indeed is already happening https://twitter.com/chrispederick/status/892768218162487300)