fix: use Code.ensure_compiled! at compile time

While investigating the changes in this PR:
elixir-lang/elixir#14451

I encountered an error about not being able to load a module at
this line, implying this is called at compile time.

@josevalim pointed out that `Code.ensure_loaded!/1` will have no effect
at compile time, and so this was really just relying on compile order
and would be non-deterministic. `Code.ensure_compiled!/1` is a superset
of `Code.ensure_loaded!/1` and will work at compile time to ensure the depended
on module is compiled.

This would appear to solve absinthe-graphql#1351.

Author: zachdaniel

lib/absinthe/schema.ex

@@ -365,7 +365,7 @@ defmodule Absinthe.Schema do
   def apply_modifiers(pipeline, schema, opts \\ []) do
     Enum.reduce(schema.__absinthe_pipeline_modifiers__(), pipeline, fn
       {module, function}, pipeline ->
-        Code.ensure_loaded!(module)
+        Code.ensure_compiled!(module)
 
         cond do
           function_exported?(module, function, 1) ->
@@ -379,7 +379,7 @@ defmodule Absinthe.Schema do
         end
 
       module, pipeline ->
-        Code.ensure_loaded!(module)
+        Code.ensure_compiled!(module)
 
         cond do
           function_exported?(module, :pipeline, 1) ->