Fix import_types/2 for A.{B, C}

[maartenvanvliet]

Fixes #937

I can reproduce issue #937 with a repo with absinthe as a dependency but not from within a test in absinthe. Has to do with the modules not being compiled (yet)

Changing https://github.com/absinthe-graphql/absinthe/blob/master/lib/absinthe/schema/notation.ex#L1683 to if Code.ensure_compiled(type_module) do fixed it. (See #534 for dealing with this in an earlier version).

Or, removing the if-clause entirely and relying on the TypeImports phase to handle this. This is what happens for ordinary imports like import_types(MyApp.Schema.Types.UnknownModule). I've done this in this PR and let failed imports add a phase error to the schema.

This could also be extended that whenever non absinthe modules are imported another error is added e.g.

defmodule Test do
end
defmodule Schema do
   use Absinthe.Schema
   import_types Test
end

The TypeImports phase could check function_exported(Test, :__absinthe_blueprint__, 0) and add an error if it's not present.

[binaryseed]

If you inspect(module) it'll get rid of the Elixir. prefix in the output...

[binaryseed]

This is great!