[curious_man]

Sure, this is a solution. But my confusion remains: why I should't put all these dependencies in the repository with the code that uses them? In the end, doing so I have (almost) all I need to produce the intended software artifact.

[undees]

There are a few reasons to do this. One is to at least nominally separate code that's licensed differently than yours (something your lawyers may ask you to do). Another is to make it easier to share your tweaks to the third-party code among several of your own projects.