Support root level map keys in map sources

[lukasbindreiter]

Checklist

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What problem does this solve?

This issue is related to this one I opened here: urfave/cli-altsrc#14

I noticed the same issue of root keys not being considered also occurs in urfave/cli itself:

Example:

mapSource := cli.NewMapSource("config", map[any]any{
	"host": "localhost",
	"database": map[any]any{
		"url": "example.com",
	},
})

cmd := &cli.Command{
	Name: "Example",
	Action: func(ctx context.Context, cmd *cli.Command) error {
		fmt.Println("Host:")
		fmt.Println(cmd.String("host"))
		fmt.Println("Database URL:")
		fmt.Println(cmd.String("database-url"))
		return nil
	},
	EnableShellCompletion: true,
	Flags: []cli.Flag{
		&cli.StringFlag{
			Name:    "host",
			Sources: cli.NewValueSourceChain(cli.NewMapValueSource("host", mapSource)),
		},
		&cli.StringFlag{
			Name:    "database-url",
			Sources: cli.NewValueSourceChain(cli.NewMapValueSource("database.url", mapSource)),
		},
	},
}

cmd.Run(context.Background(), os.Args)

will output (host is empty):

Host:

Database URL:
example.com

Solution description

I want the above snipped to set host to localhost and database-url to example.com, instead of only setting the database url.

The issue is located here - because it always returns nil if there is no dot in a key:
https://github.com/urfave/cli/blob/main/value_source.go#L196