Investigate provider hook for cycle detection

[uranusjr]

Molinilo (CocoaPods) detects dependency cycles and rejects them. ResolveLib does not do this (neither does pip currently), but maybe it would be possible to implement a provider hook to let the user decide whether or not to?

[pradyunsg]

Noting from https://pip.pypa.io/en/stable/reference/pip_install/#installation-order:

In the event of a dependency cycle (aka “circular dependency”), the current implementation (which might possibly change later) has it such that the first encountered member of the cycle is installed last.