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/util.go

@@ -279,6 +279,6 @@ func jsonType(v reflect.Value) (string, bool) {
 
 func assert(cond bool, msg string) {
 	if !cond {
-		panic(msg)
+		panic("assertion failed: " + msg)
 	}
 }