Embedded struct with prefix does not work well with `kong.ConfigFlag`

The following examples:

```go
package main

import (
"fmt"

"github.com/alecthomas/kong"
)

func main() {
var cli1 struct {
Module struct {
ConfigFile kong.ConfigFlag
Val1 int
Val2 int
} `embed:"" prefix:"module."`
// other params
}

kong.Parse(&cli1, kong.Configuration(kong.JSON))

fmt.Println(cli1)

var cli2 struct {
Module struct {
ConfigFile kong.ConfigFlag
Module struct {
Val1 int
Val2 int
} `embed:"" prefix:"module."`
} `embed:"" prefix:"module."`
// other params
}
kong.Parse(&cli2, kong.Configuration(kong.JSON))

fmt.Println(cli2)
}
```

With a config file that looks like

```json
{
"val1": 1,
"val2": 2
}
```

When run with the command
`go run main.go --module.config-file ./config.json`

Will output:
```
{{./config.json 0 0}}
{{./config.json {0 0}}}
```

Am I doing something wrong ? Or this is a bug/side effect ?