And yet that's the right approach. It's not really copying but rather onboarding.

You don't want to depend on a third-party hosting the code, so you need to copy it, and pin it to a specific version. You might also need to patch it since you'll be using it and most likely will run into problems with it.

Using third-party code means taking ownership of all the problems one might encounter when trying to use it in your project, so you might as well just adapt it for it to work with your tools and processes.

If you use a modular system this is essentially just maintaining a fork.