Fixes #22
Replaces #24

From my reading of the docs and other research, it looks like Code.ensure_loaded? is a better way to check whether an optional dependency module exists or not. Details below. This PR preserves the intention of only including Absinthe-related code when Absinthe is present in the parent project. #24 was a temporary fix to disable the check as I understand it.

This also fixes a formatting-related test failure in the doc tests for assertions/absinthe.ex. I still get 3 test failures in the doc tests (with Elixir 1.12.3), but I'm not sure what the problem is or how to fix them. The errors all happen in format_fields with the message:

** (Protocol.UndefinedError) protocol Enumerable not implemented for :scalar of type Atom. This protocol is implemented for the following type(s): HashSet, Range, Map, Function, List, Stream, Date.Range, HashDict, GenEvent.Stream, MapSet, File.Stream, IO.Stream

Details:

• The Elixir docs for Code.ensure_compiled say:

  Given this function halts compilation, use it carefully. In particular, avoid using it to guess which modules are in the system.

• In an Elixir Forum post, Jose Valim suggests using Code.ensure_loaded? to check if a dependency is loaded or not.