Support relative base URIs

See: sourcemeta/jsonschema#185
Signed-off-by: Juan Cruz Viotti <jv@jviotti.com>

test/evaluator/evaluator_draft4_test.cc

@@ -5658,3 +5658,23 @@ TEST(Evaluator_draft4, ref_to_non_schema) {
     FAIL();
   }
 }
+
+TEST(Evaluator_draft4, relative_base_uri_with_ref) {
+  const sourcemeta::jsontoolkit::JSON schema{
+      sourcemeta::jsontoolkit::parse(R"JSON({
+    "$schema": "http://json-schema.org/draft-04/schema",
+    "id": "common",
+    "allOf": [ { "$ref": "#reference" } ],
+    "definitions": {
+      "reference": {
+        "id": "#reference",
+        "type": "string"
+      }
+    }
+  })JSON")};
+
+  const sourcemeta::jsontoolkit::JSON instance{"test"};
+  EVALUATE_WITH_TRACE_FAST_SUCCESS(schema, instance, 1);
+
+  // TODO: Assert on traces
+}