Fix the handling of references in JSON schemas when validating

References must be resolved when loading the schema, otherwise they will fail
when using them.
The easiest way is actually to let the JsonSchema library load the file itself.

composer.json

@@ -23,7 +23,7 @@
     },
     "require": {
         "php": "^5.3.2 || ^7.0",
-        "justinrainbow/json-schema": "^1.6 || ^2.0 || ^3.0 || ^4.0",
+        "justinrainbow/json-schema": "^3.0 || ^4.0",
         "composer/ca-bundle": "^1.0",
         "composer/semver": "^1.0",
         "composer/spdx-licenses": "^1.0",

composer.lock

@@ -4,7 +4,7 @@
         "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#composer-lock-the-lock-file",
         "This file is @generated automatically"
     ],
-    "content-hash": "deb4df08cdd39eac7d11880586076ba1",
+    "content-hash": "e18501d127e13e3619f80abbcf372c81",
     "packages": [
         {
             "name": "composer/ca-bundle",

src/Composer/Json/JsonFile.php

@@ -157,7 +157,13 @@ class JsonFile
         }
 
         $schemaFile = __DIR__ . '/../../../res/composer-schema.json';
-        $schemaData = json_decode(file_get_contents($schemaFile));
+
+        // Prepend with file:// only when not using a special schema already (e.g. in the phar)
+        if (false === strpos($schemaFile, '://')) {
+            $schemaFile = 'file://' . $schemaFile;
+        }
+
+        $schemaData = (object) array('$ref' => $schemaFile);
 
         if ($schema === self::LAX_SCHEMA) {
             $schemaData->additionalProperties = true;