Fix property groups overriding real properties

[paulloz]

With the following script, each rebuild reverts the exported value to its default.

public partial class Foo : Node
{
    [ExportGroup("Test")]
    [Export]
    private string Test;
}

The same thing happens in GDScript. With the following script, modifying the default value in the code overwrites any value defined through the inspector.

extends Node

@export_group("test")
@export
var test : String = "Hello World!"

It is triggered by the group using the same name as the actual property (changing the group names above prevents the weird behaviour).

[YuriSizov]

Makes sense, I think.

[akien-mga]

Thanks!

[YuriSizov]

Cherry-picked for 4.2.1.