Parse composed schema into a queryable form

> This is a high-level tracking issue. The implementation may surface in a number of PRs, and even this issue may be further broken down. It relates to #61 and https://github.com/apollographql/apollo-server/issues/4263, though I believe only in the sense that the final product would need to use the decided-upon/implemented directives.

At a high level, this aims to build utilities to operate on a composed schema (as in, a schema which has been annotated with composition directives, as referred to in #61 and https://github.com/apollographql/apollo-server/issues/4263). A small example would be to, e.g., locate elements of the schema with a particular name, of a particular type, etc.

This is a high-level tracking issue with no files, tests, or entry points named. Start by reading #61 and the linked Apollo Server issue to understand the composition directives and intended final product. Done would mean utilities can operate on a composed schema and locate elements by name or type, but the implementation scope is explicitly expected to be broken into further work.

Written by the indexing model from the issue text.