Allow other expansion sources: services, kgx

currently an ontology file can be used as expansion source (closures, id<->name/syns, etc)

we should also allow

- a kgx file (nodes for id/names, edges for closures or equivalencies)
- services
- biothings
- sri normalizer (cc @cbizon)

for the services, implement caching to avoid hammering

No files, tests, or entry points are named. Start by locating the existing ontology-file expansion source, then trace how sources are selected and cached; done means KGX files and the Biothings and SRI Normalizer services can provide expansions without repeated service requests.

Written by the indexing model from the issue text.