jsonschema: resolve refs

Implement the $ref keyword, except for remote references (those
pointing outside the root schema).

Change-Id: Ia5ca88464e62e9c613970d8d9fcd1512ab288934
Reviewed-on: https://go-review.googlesource.com/c/tools/+/670775
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Jonathan Amsterdam <jba@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

jsonschema/schema.go

@@ -115,8 +115,11 @@ type Schema struct {
 	// The parent base URI at top level is where the schema was loaded from, or
 	// if not loaded, then it should be provided to Schema.Resolve.
 	baseURI *url.URL
+	// The schema to which Ref refers.
+	resolvedRef *Schema
 	// map from anchors to subschemas
-	anchors           map[string]*Schema
+	anchors map[string]*Schema
+	// compiled regexps
 	pattern           *regexp.Regexp
 	patternProperties map[*regexp.Regexp]*Schema
 }