jsonschema: resolve anchors

Change-Id: Ibe7d8df0a46d3e3a8b5d0651b1113a54e756c7df
Reviewed-on: https://go-review.googlesource.com/c/tools/+/669998
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

jsonschema/schema.go

@@ -114,7 +114,9 @@ type Schema struct {
 	// Otherwise, the base URI is the ID resolved against the parent's baseURI.
 	// 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
+	baseURI *url.URL
+	// map from anchors to subschemas
+	anchors           map[string]*Schema
 	pattern           *regexp.Regexp
 	patternProperties map[*regexp.Regexp]*Schema
 }