You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: source.go
    path of file: ./repos/cosmos-sdk/core/genesis
    
    the code of the file until where you have to start completion: package genesis

import (
	"bytes"
	"encoding/json"
	"io"

	"cosmossdk.io/core/appmodule"
)

// SourceFromRawJSON returns a genesis source based on a raw JSON message.
func SourceFromRawJSON(
    
func SourceFromRawJSON(message json.RawMessage) (appmodule.GenesisSource, error) {
	var m map[string]json.RawMessage
	err := json.Unmarshal(message, &m)
	if err != nil {
		return nil, err
	}
	return func(field string) (io.ReadCloser, error) {
		j, ok := m[field]
		if !ok {
			return nil, nil
		}
		return readCloserWrapper{bytes.NewReader(j)}, nil
	}, nil
}