Fix infinite loop when creating a graph for a project with cyclic dependencies

[Veetaha]

Fixes #184

I am not well acquainted with the codebase. Not sure how those KrateSpans are connected with bans::check, just created them analogously to existing tests.
Ideally, there should be some integration tests infra. I hand-rolled a test project in tests/test_data/cyclic_dependencies just for this case.

cc @Jake-Shadle

[Veetaha]

I separated a test and a fix into two commits so that is evident that the test fails without the fix.
I am still not sure the fix will work in 100%, looks like the hash sets were created to handle this problem?
Anyway, after my amendments, cargo deny works fine with my initial project and with the test project too:

[Jake-Shadle]

Yes, the hashset was meant to avoid this problem, but I think it might have gotten lost in the last refactor (or never worked!)

This was a great PR though, thanks for both adding a test and splitting it between broken and fixed!

[Jake-Shadle]

This was published in the 0.6.7 release, thanks again!