fix: fix pipeline I/O detection

[ZanSara]

The introduction of the Variadic type and some older bugs were affecting the correctness of Pipeline's input and output detection.

This PR:

• Renames sender to senders
• Introduces a mirror consumers field to ease access to the consumers of a specific socket's value
• Keeps Variadic inputs into consideration when looking for pipeline inputs
• Changes the logic of pipeline's outputs detection to account for nodes that output only part of their results

---

Note: this breaks a lot of tests. Fixing this implies modifications in the run() method that will be addressed in separate PRs.