[GuB-42]

If these libraries are so small, self-contained and "completed", why not just copy-paste these functions?

Submodules can work too, but do you really need these extra lines in your build scripts, extra files and directories, and the import lines just for a five line function? Copy-pasting is much simpler, with maybe a comment referring to the original source.

Note: there may be some legal reasons for keeping "micro-libraries" separate, or for not using them at all though but IANAL as they say.

[5Qn8mNbc2FNCiVV]

As soon as source code is in your repo it's way more probable to getting touched. I'd never open that box ever because I don't want to waste time with my team touching code that they shouldn't when reviewing.

If you want the same functionality, build it according to the conventions in the codebase and strip out everything else that isn't required for the exact use case (since it's not a library anymore)