Follow-up MR for the imp module

As I was trying to test my MR for Meson build support ( !44 (merged)), I still got a lot of error messages related to the imp module, even after cherry-picking @tyrylu 's commits. Looking at the error messages, I found out we still had one usage of imp in "plugins/validate.py", so I ported that too. Since I couldn't push to their branch, I just made this follow-up MR with the commit cherr-picked.