Self-referencing collections shouldn't have to have "using IOG;"

[AArnott]

Given an immutable type with a self-referencing collection member:

    [GenerateImmutable]
    public partial class X
    {
        [DebuggerBrowsable(DebuggerBrowsableState.Never)]
        private readonly ImmutableArray<X> children;
    }

This produces a compile error in the generated code unless the above source file includes using ImmutableObjectGraph; at the top, since the generated code doesn't fully-qualify some types.