Re-vendor, and use mtrmac/image-spec:id-based-loader to fix tests

Anyone running (vndr) currently ends up with failing tests in OCI schema
validation because gojsonschema has fixed its "$ref" interpretation, exposing
inconsistent URI usage inside image-spec/schema.

So, this runs (vndr), and uses mtrmac/image-spec:id-based-loader
( https://github.com/opencontainers/image-spec/pull/739 ) to make the tests pass
again.  As soon as that PR is merged we should revert to using the upstream
image-spec repo again.

vendor/github.com/opencontainers/image-spec/schema/loader.go

@@ -0,0 +1,126 @@
+// Copyright 2018 The Linux Foundation
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package schema
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"net/http"
+	"strings"
+
+	"github.com/xeipuuv/gojsonreference"
+	"github.com/xeipuuv/gojsonschema"
+)
+
+// fsLoaderFactory implements gojsonschema.JSONLoaderFactory by reading files under the specified namespaces from the root of fs.
+type fsLoaderFactory struct {
+	namespaces []string
+	fs         http.FileSystem
+}
+
+// newFSLoaderFactory returns a fsLoaderFactory reading files under the specified namespaces from the root of fs.
+func newFSLoaderFactory(namespaces []string, fs http.FileSystem) *fsLoaderFactory {
+	return &fsLoaderFactory{
+		namespaces: namespaces,
+		fs:         fs,
+	}
+}
+
+func (factory *fsLoaderFactory) New(source string) gojsonschema.JSONLoader {
+	return &fsLoader{
+		factory: factory,
+		source:  source,
+	}
+}
+
+// refContents returns the contents of ref, if available in fsLoaderFactory.
+func (factory *fsLoaderFactory) refContents(ref gojsonreference.JsonReference) ([]byte, error) {
+	refStr := ref.String()
+	path := ""
+	for _, ns := range factory.namespaces {
+		if strings.HasPrefix(refStr, ns) {
+			path = "/" + strings.TrimPrefix(refStr, ns)
+			break
+		}
+	}
+	if path == "" {
+		return nil, fmt.Errorf("Schema reference %#v unexpectedly not available in fsLoaderFactory with namespaces %#v", path, factory.namespaces)
+	}
+
+	f, err := factory.fs.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+
+	return ioutil.ReadAll(f)
+}
+
+// fsLoader implements gojsonschema.JSONLoader by reading the document named by source from a fsLoaderFactory.
+type fsLoader struct {
+	factory *fsLoaderFactory
+	source  string
+}
+
+// JsonSource implements gojsonschema.JSONLoader.JsonSource. The "Json" capitalization needs to be maintained to conform to the interface.
+func (l *fsLoader) JsonSource() interface{} { // nolint: golint
+	return l.source
+}
+
+func (l *fsLoader) LoadJSON() (interface{}, error) {
+	// Based on gojsonschema.jsonReferenceLoader.LoadJSON.
+	reference, err := gojsonreference.NewJsonReference(l.source)
+	if err != nil {
+		return nil, err
+	}
+
+	refToURL := reference
+	refToURL.GetUrl().Fragment = ""
+
+	body, err := l.factory.refContents(refToURL)
+	if err != nil {
+		return nil, err
+	}
+
+	return decodeJSONUsingNumber(bytes.NewReader(body))
+}
+
+// decodeJSONUsingNumber returns JSON parsed from an io.Reader
+func decodeJSONUsingNumber(r io.Reader) (interface{}, error) {
+	// Copied from gojsonschema.
+	var document interface{}
+
+	decoder := json.NewDecoder(r)
+	decoder.UseNumber()
+
+	err := decoder.Decode(&document)
+	if err != nil {
+		return nil, err
+	}
+
+	return document, nil
+}
+
+// JsonReference implements gojsonschema.JSONLoader.JsonReference. The "Json" capitalization needs to be maintained to conform to the interface.
+func (l *fsLoader) JsonReference() (gojsonreference.JsonReference, error) { // nolint: golint
+	return gojsonreference.NewJsonReference(l.JsonSource().(string))
+}
+
+func (l *fsLoader) LoaderFactory() gojsonschema.JSONLoaderFactory {
+	return l.factory
+}