ModelUtils.getUnusedSchemas reports simple model from array types as unused

Description

Found in #251

openapi-generator version

3.0.0

OpenAPI declaration file content or url

https://github.com/OAI/OpenAPI-Specification/blob/master/examples/v3.0/petstore.yaml

Command line used for generation

n/a (not a generation bug)

Steps to reproduce

Discovered while testing #251.

On that branch:

mvn clean install
cd modules/openapi-generator-gradle-plugin/
./gradlew assemble publishToMavenLocal
./gradlew --build-file=samples/local-spec/build.gradle generateGoWithInvalidSpec

Related issues/PRs

• #251

Suggest a fix/enhancement

ModelUtils.getUnusedSchemas calls into visitOpenAPI, and visitOpenAPI needs to account for multiple levels of nested arrays, enums, map structures, and polymorphic types.