There are definitely 100-line chunks of code I wouldn't want to rewrite from scratch. They also tend not to be the sort of thing that needs a lot of updates, so a copy/paste job ought to do the job.

The big advantage with a dependency manager is that you don't have to find all of the dependency's dependencies, figure out the right build settings, etc. That's super helpful when it's huge, but it's not really doing anything for you when it's small.