Improve dependency management for higher-level rlay-ontology clients

[MichaelHirn]

With the arrival of v0.2.0 it becomes easier (recommended) to separate application ontologies into their own npm package which exposes (if it should be reused in other packages) their auto-generated client. It is also custom behavior that these rlay-ontology packages expose their own classes which provide higher-level interfaces to their ontology. Example rlay-ontology package here: https://github.com/MichaelHirn/rlay-ontology-airtable

However, to consume these rlay-ontology packages i.e. require them in rlay-ontology packages and applications and use their ontology and high-level interfaces, is non trivial and requires manually changing the auto-generated client of the application.

To prevent that, we need to change two things:

• integrating the schema of rlay-ontology packages
• integrating the higher-level interfaces of rlay-ontology packages

Integrating the schema of rlay-ontology packages

The proposed solution is to update rlay-seed to produce an index of entityKey => entityPayload instead of entityKey => entityCID as it is currently implemented; an update of rlay-generate to only require the output of rlay-seed (and no path to any ontology file) or even work on the input of rlay-seed. Then rlay-ontology packages have to expose their src/ontology.js file which module.exports a rlay-seed JSON object. In the application that ontology JSON object can then be required and dropped into the imports attribute of its own rlay-seed JSON object. rlay-seed needs to accept payloads instead of only cids for its imports attribute.

If you want your rlay-ontology package to be required by other packages, you export the output of rlay-seed (aka. the ontology file's rlay-seed JSON object), otherwise don't.

Integrating the higher-level interfaces of rlay-ontology packages

A bit more unclear but orientate ourselves on plugin systems like those from babel, eslint, etc.

cc @hobofan