fix(flatten): allows the use of pointers to schemas in x-... extensions

[fredbi]

This PR allows for reusing schemas stored in x-... extensions of the spec, via json pointers.

We have seen some specs using such extensions to create "alternate definitions". The spec flattening canonicalizes such schemas by reintroducing them as #/definitions, so they can be consumed by codegen.

• requires: fix(GetForToken): added support for pointer to interface{} jsonpointer#16
• contributes: swagger generate client command fails with invalid token reference go-swagger/go-swagger#1898

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b9463f9) 83.37% compared to head (887ddcd) 92.68%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #89      +/-   ##
==========================================
+ Coverage   83.37%   92.68%   +9.30%     
==========================================
  Files          14        7       -7     
  Lines        2490     1899     -591     
==========================================
- Hits         2076     1760     -316     
+ Misses        364      101     -263     
+ Partials       50       38      -12     

Flag	Coverage Δ
oldstable	92.68% <100.00%> (+9.30%)	⬆️
stable	92.68% <100.00%> (+9.30%)	⬆️

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[fredbi]

Here is the spec from go-swagger/go-swagger#1898 used for testing this feature.
Before this patch, the x-... section unmarshals as an interface{}
and therefore cannot be properly processed with $ref and schema.
With the new flattening, I could test (manually) that go-swagger generates a proper model for this additional schema and generates a proper server and client (more tests on the follow-up PR on go-swagger to include this change).

{
  "swagger": "2.0",
  "info": {
    "title": "example.proto",
    "version": "version not set"
  },
  "schemes": [
    "http",
    "https"
  ],
  "consumes": [
    "application/json"
  ],
  "produces": [
    "application/json"
  ],
  "paths": {
    "/example/v2/GetEvents": {
      "get": {
        "operationId": "GetEvents",
        "responses": {
          "200": {
            "description": "A successful response.(streaming responses)",
            "schema": {
              "$ref": "#/x-stream-definitions/v2EventMsg"
            }
          }
        },
        "parameters": [
          {
            "name": "afterEventID",
            "description": ".",
            "in": "query",
            "required": false,
            "type": "string"
          }
        ],
        "tags": [
          "Matchmaking"
        ]
      }
    }
  },
  "definitions": {
    "protobufAny": {
      "type": "object",
      "properties": {
        "type_url": {
          "type": "string"
        },
        "value": {
          "type": "string",
          "format": "byte"
        }
      }
    },
    "runtimeStreamError": {
      "type": "object",
      "properties": {
        "grpc_code": {
          "type": "integer",
          "format": "int32"
        },
        "http_code": {
          "type": "integer",
          "format": "int32"
        },
        "message": {
          "type": "string"
        },
        "http_status": {
          "type": "string"
        },
        "details": {
          "type": "array",
          "items": {
            "$ref": "#/definitions/protobufAny"
          }
        }
      }
    },
    "v2EventMsg": {
      "type": "object",
      "properties": {
        "eventID": {
          "type": "string"
        }
      }
    }
  },
  "x-stream-definitions": {
    "v2EventMsg": {
      "type": "object",
      "properties": {
        "result": {
          "$ref": "#/definitions/v2EventMsg"
        },
        "error": {
          "$ref": "#/definitions/runtimeStreamError"
        }
      },
      "title": "Stream result of v2EventMsg"
    }
  }
}