Latter half of domain is cut off resource group

This means `kustomize.toolkit.fluxcd.io/v1beta2` will just end up as `kustomize`. This seems like strange behaviour, and breaks assumptions about multiple resource types co-existing with each other.

I've traced it to https://github.com/instrumenta/kubeval/commit/acbe5159c89d68d078c653b8c6083fb0a2637352. @garethr perhaps you could give some insight here?