rf(schema): Consolidate rules.dataset_metadata under rules.json

[effigies]

This is a slight cleanup. rules.dataset_metadata and rules.json developed in parallel, but are ultimately the same thing. This makes it easier on schema users to find the rules that apply to any given JSON file by consolidating all under rules.json.

I did move the dataset_description_with_genetics rule to rules.json.genetics instead of rules.json.dataset, since those rules were introduced in the genetics BEP. It could also be argued that it should go under rules.json.dataset, since it directly impacts dataset_description.json. The location has no impact on validation, it's just a question of what's most intuitive.

[bendhouseart]

Agree, it's slightly more intuitive.

[effigies]

Tempting @yarikoptic into reviewing with a promise of a more consistent schema.

[yarikoptic]

doesn't this change/break metaschema and thus requiring boost of https://github.com/bids-standard/bids-specification/blob/master/src/schema/SCHEMA_VERSION to 2.0.0-dev?

[effigies]

It seemed like a relatively small schema change, so I figured the minor version bump would be fine, but I have no objection to calling it major.