The defense is staged deployment and active users. This obviously depends on the blutness of the malicious code.
If I may assume easily noticed effects of the malicious code: A dev at our place - using java with maven - would update the library, his workstation would get owned. This could have impacts, but if we notice, we'd wipe that workstation, re-image from backup and get in contact with sonatype to kill that version. This version would never touch staging, the last step before prod.
If we don't notice on the workstation, there's a good chance we or our IDS would notice trouble either on our testing servers or our staging servers, since especially staging is similar to prod and subject to load tests similar to prod load. Once we're there, it's back to bug reports with the library and contact with sonatype to handle that version.
If we can't notice the malicious code at all until due to really really smart activation mechanisms... well then we're in NSA conspiracy land again.