Resolve schema modules in discriminator mapping

[mbuhot]

fixes #387

Given a schemas such as PetAppointmentRequest from the test suite:

defmodule PetAppointmentRequest do
    OpenApiSpex.schema(%{
      title: "PetAppointmentRequest",
      description: "POST body for making a pet appointment",
      type: :object,
      oneOf: [
        TrainingAppointment,
        GroomingAppointment
      ],
      discriminator: %OpenApiSpex.Discriminator{
        propertyName: "appointment_type",
        mapping: %{
          "training" => TrainingAppointment,
          "grooming" => GroomingAppointment
        }
      }
    })
  end

The resolve_schema_modules function will now replace modules appearing in the discriminator.property.mapping map, with the appropriate path to the schema, eg "#/components/schemas/TrainingAppointment".

Rendered to JSON gives:

      "PetAppointmentRequest": {
        "description": "POST body for making a pet appointment",
        "discriminator": {
          "mapping": {
            "grooming": "#/components/schemas/GroomingAppointment",
            "training": "#/components/schemas/TrainingAppointment"
          },
          "propertyName": "appointment_type"
        },
        "oneOf": [
          {
            "$ref": "#/components/schemas/TrainingAppointment"
          },
          {
            "$ref": "#/components/schemas/GroomingAppointment"
          }
        ],
        "title": "PetAppointmentRequest",
        "type": "object"
      },