Saved object types are incorrectly determined

[legrego]

Kibana version: 7.0

Saved Object References introduced a new root property (references) to the Kibana index mapping. This mapping happens to satisfy the existing criteria used to derive the full list of available saved object types. This full list of types is exposed on the saved objects service for consumers, and is used in a number of places.

The criteria for determining valid saved object types should be updated so that we do not include references, or anything else that we attach to the mapping which is not a saved object type.

We caught this due to a CI failure in our feature branch following a recent merge from master, so it might be worthwhile to also add a test to ensure this continues to work in the future.

/cc @kobelb

[elasticmachine]

Pinging @elastic/kibana-platform