bug#51021: detect loops in module/package graph

[raingloom]

I'll be short and blunt, currently it sucks big time whenever you have
a loop in your packages.
There is no indication other than Guix hanging forever without any
output.

I don't want to spend my time manually finding loops in graphs,
computers are better at that.

Sadly I don't know when I'll have time to implement this, so if someone
knows of a solution, they should not hesitate with sending a patch and
making all our lives easier.