depinst option to detect cycles

[grisumbras]

This PR adds an option to depinst to detect cycles for the topmost library. A detected cyle is treated like an error (an exception is raised).

[pdimov]

Looks like this doesn't do anything when a cycle is detected, but just crashes with an unhandled exception?

[grisumbras]

Yes, my idea was that you add the flag and then your CI jobs fail if you get a cycle. Python will print the exception message which contains the cycle description.

[pdimov]

I'm not entirely convinced of the use case for this option, but I suppose it could be useful in a scenario when a cycle is introduced somewhere and then removed; this can then be added to CI to prevent the problem from reappearing.

Or perhaps you could make a PR to e.g. Geometry with the option added to demonstrate that a cycle is present.

[grisumbras]

Well, we can advertise this new option. I can also make the flag to default to True.

Or do you have an idea for an alternative approach?

[pdimov]

No, I don't want it to default to True. depinst works perfectly well when cycles are present.