no support for definitions

[Jonathan Heindl]

a schema like

{
    "id": "http://json-schema.org/geo",
    "$schema": "http://json-schema.org/draft-06/schema#",
    "description": "A geographical coordinate",
    "type": "object",
    "definitions": {
        "Def1": {
            "type": "object",
            "properties": {
                "longitude": {
                    "type": "string",
                    "const": "sdf"
                }
            }
        }
    },
    "properties": {
        "latitude": {
            "$def": "#/definitions/Def1"
        },
        "longitude": {
            "type": "string",
            "const": "sdfss"
        }
    }
}

should be resolvable