Create a reference library to easily query custom elements info

We want to avoid duplication of data in the schema, but we also want to allow for easily querying a schema, or related collection of schemas.

Tools will want to answer questions like:
- What are all the elements in this package?
- Build an index from tag name to class, or class to tag name
- What is the entire class hierarchy for class A?
- Find all references to a symbol

We can make this much easier by building a shared library that given a set of package docs builds a queryable model from them. It should not live in this repository, but the people here would be interested in it.