There is some variation in jsonnet "result" conventions.  This change
assumes the top level is a JSON object, and then walks down looking
for objects with `apiVersion` & `kind` fields.

While walking down, the current code will expand JSON arrays, and
object values.

In particular, this should correctly accept:
- single-value JSON files
- arrays (jsonnet --yaml-stream)
- "filename->object" objects (jsonnet --multi)

(Note that any v1.Lists encountered are still expanded in a
second-pass, as before)