Prevent loops in the dependency graph

[Tiagoperes]

There's nothing testing for cycles in the dependency graph. What happens when a cycle is detected? Does it start an infinite loop? Does the second appearance of an object gets ignored?

1. Write a test
2. Ignore the second time an object appears if it's currently entering an infinite loop.

Attention: the entries Nimbus itself creates in the dependency graph will never have cycles, but since this is an open API and new nodes can be added from anywhere, we should be able to detect possible errors introduced by the user.