Fix introspection query failing with "Cannot return null for non-nullable field"

[dhrdlicka]

The introspection query is currently failing due to argument types for the @link directive not resolving properly, as they don't seem to get actually imported from Absinthe.Federation.Schema.Prototype.

This PR adds a import_type statement to fix that.

[kdawgwilk]

LGTM! @kzlsakal you want to take a look and merge if it looks good to you?

[kzlsakal]

Thanks for the PR @dhrdlicka!

I tested this on a local subgraph, and introspection on both the GraphiQL Playground and the Apollo Explorer are producing the following error, although the network call returns a successful response:

instrument.js:105 Error: Invalid or incomplete schema, unknown type: _FieldSet. Ensure that a full introspection query is used in order to build a client schema.

Do we need to omit certain records that refer to this _FieldSet? Not sure if this change is causing redundant types to be added to the schema or if they were already there, but it seems like we still cannot produce a successful introspection the way it is.

[dhrdlicka]

@kzlsakal I guess I should have checked the actual query result before coming to premature conclusions 😄

Yeah, the introspection query doesn't crash anymore but the types defined in the schema prototype are still not there. I think they might be getting stripped when they're not actually used in the schema itself.

[dhrdlicka]

I opened an issue in the Absinthe repository for this problem: absinthe-graphql/absinthe#1279