resolution doesn't seem to handle $ref at the root

[buchanae]

Describe the bug
I think the jsonschema for the helm values in the cert-manager chart may demonstrate a bug with an edge case in resolution where the root object has a "$ref".

https://github.com/cert-manager/cert-manager/blob/master/deploy/charts/cert-manager/values.schema.json

To Reproduce
Something like:

package main

import (
	"encoding/json"
	"os"

	"github.com/google/jsonschema-go/jsonschema"
)

func main() {
	schemaData, _ := os.ReadFile("values.schema.json")

	var schema jsonschema.Schema
	json.Unmarshal(schemaData, &schema)

	// This is the workaround I used:
	// if strings.HasPrefix(schema.Ref, "#/$defs/") {
	// 	key := strings.TrimPrefix(schema.Ref, "#/$defs/")
	// 	def, ok := schema.Defs[key]
	// 	if ok {
	// 		root := schema
	// 		schema = *def
	// 		schema.Defs = make(map[string]*jsonschema.Schema)
	// 		for k, v := range root.Defs {
	// 			if k != key {
	// 				schema.Defs[k] = v
	// 			}
	// 		}
	// 	}
	// }

	resolved, err := schema.Resolve(&jsonschema.ResolveOptions{})
	if err != nil {
		panic(err)
	}
	if resolved.Schema().Type != "object" {
		panic("bug")
	}
	println("ok")
}

That workaround only seemed to work at the root level. The child properties (e.g. root.properties.acmesolver) were still unresolved for some reason.