[5.x] Prevents Recursive Fieldsets

[JohnathonKoster]

This PR closes #1725 by returning early if we've already seen a fieldset's handle. This PR targets 5.x since it changes some constructions and public method signatures,

Currently it logs a warning instead of throwing an exception, but this could be easily changed. If this direction looks correct, I will work on getting the UI updated to also not attempt to recursively process includes (this could also be sidestepped by just throwing an exception).

Will add docs issue depending on which direction is desired here (log and fix UI, just throw an exception, no change for now) 🙂

[duncanmcclean]

Since this PR is for v5, I've changed the target branch to master. Would you be able to rebase your branch with the latest changes from master?