Prevent infer input fields infinite recursion

[jasonphillips]

Test and fix for #2420

Ensures that the recursive type-checking of new graphql fields added by plugins does not infinitely recur when types contain circular references. The solution used is to simply track the set of types during a traversal, aborting at the first instance of circularity, yet preserving the functionality of input type inference up until that point.

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[KyleAMathews]

Thanks!

Seems correct. @Zalastax any feedback?

[Zalastax]

Very nice! Merge it!

It's great that since createTypeNames guarantees unique names this won't run into conflicts even if two fields have the same name.
When gatsby start using React 16 I'd consider using Set, but that's for the future.

[KyleAMathews]

This is node.js code so we're already good there :-) Set was first added to Node 4 which is the lowest version we support.

[KyleAMathews]

But anyways, this is plenty good enough. Merging! Thanks!

[jlengstorf]

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