title: JSON schema $ref does not work for relative path
body: <p>I have 3 schemas:</p>
  
  <p>child schema:</p>
  
  <pre><code>{
      "title": "child_schema",
      "type": "object",
      "properties": {
          "wyx":{
              "type": "number"
          }
       },
      "additionalProperties": false,
      "required": ["wyx"]
  }
  </code></pre>
  
  <p>parent schema:</p>
  
  <pre><code>{
      "title": "parent",
      "type": "object",
      "properties": {
          "x": {
              "type": "number"
          },
          "y": {
              "type": "number"
          },
          "child": {
              "$ref": "file:child.json"
          }
      }
  }
  </code></pre>
  
  <p>grandpa schema:</p>
  
  <pre><code>{
      "type": "object",
      "title": "grandpa",
      "properties": {
          "reason": {
              "$ref": "file:parent.json"
          }
      },
      "additionalProperties": false
  }
  </code></pre>
  
  <p>As you can see, gradpa has a ref to parent and parent has a ref to child.
  All these 3 files are inside the same folder.
  When I use python validator to validate grandpa schema, I will keep on getting an error called RefResolutionError. </p>
  
  <p>HOWEVER, if I do not have grandpa and I just use parent schema and child schema, everything worked!! So the issue is I cannot have a ref pointing to a ref(2 levels). But I can have a ref pointing to a schema(just 1 level.)</p>
  
  <p>I wonder why </p>