Thanks @ben.boeckel! Something that could also help would be to have a practical example of an installable C++ module library that uses third parties/external dependencies, which is exactly my problem. I can try to understand the error better from a functional example, and if I find the solution, I can clarify the error here to help anyone else who faces the same problem in the future.
As it’s something new, I couldn’t find any examples in this direction.