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

**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:
```go
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.