Extension context validation on recursively defined elements does not work properly

[Kasdejong]

When running extension context validation on an element with e.g. location Questionnaire.item.item.item.item, the validator will report an error/warning if the extension is defined to only be used on "Questionnaire.item". This behaviour is incorrect. We should find a way to account for these cases.

[ewoutkramer]

This one keeps coming up (see also the discussion on our internal Slack), so let's figure this out. If there are things unclear in the spec, we can ask in May.

[alexzautke]

Blocked by discussion: https://chat.fhir.org/#narrow/channel/179166-implementers/topic/Recursive.20schemas.20validation/near/580735280