Closes #79 Fix dereference of alias schemas

[walsha2]

Closes #79 Where String schema alias was not being properly written. For the following schema:

"schemas": {
  "StringAlias": {
    "type": "string"
  },
  "MyObject": {
    "type": "object",
    "properties": {
      "myField": {
        "$ref": "#/components/schemas/StringAlias"
      }
    }
  }
}

The generator now properly creates the expected types:

typedef StringAlias = String;

@freezed
class MyObject with _$MyObject {
  const MyObject._();

  /// Factory constructor for MyObject
  const factory MyObject({
    /// No Description
    @JsonKey(includeIfNull: false) StringAlias? myField,
  }) = _MyObject;

  /// Object construction from a JSON representation
  factory MyObject.fromJson(Map<String, dynamic> json) =>
      _$MyObjectFromJson(json);

  /// List of all property names of schema
  static const List<String> propertyNames = ['myField'];

  /// Perform validations on the schema property values
  String? validateSchema() {
    return null;
  }

  /// Map representation of object (not serialized)
  Map<String, dynamic> toMap() {
    return {
      'myField': myField,
    };
  }
}

To run this test and inspect the generated code:

make test TEST_ARGS="-n Misc"