Manifest files: add support for corresponding flat and object properties

[mtojek]

Due to different implementations, some properties may be considered as faulty:

kibana.version: 1

vs.

kibana:
   version: 1

Same for: elasticsearch.index_template.mappings

The goal of this task is to research all places that are required to be adjusted in elastic-package and in package-spec.

[ycombinator]

I haven’t dug into this but it seems like something we should fix in the package spec go library? If so, should we move this issue to that repo?

[mtojek]

I'm not quite sure how is this handled by elastic-package, but if it depends on standard json.Marshal/Unmarshal, then probably it needs an adjustment too.

[mtojek]

I researched the area a bit and it looks that the only place to be affected in the elastic-package is this place:
https://github.com/elastic/elastic-package/blob/master/internal/packages/packages.go#L90

I will focus on the package-spec first and transform the issue there as you suggested.

[mtojek]

As the change is merged to package-spec I will adjust the elastic-package to support flat/object properties.