`DefineWithMethodsPerProperty = true` fails when source doc has no using for ImmutableObjectGraph

When a file wants helper methods:

```cs
[GenerateImmutable(DefineWithMethodsPerProperty = true)]
public partial class X
{
    private readonly ImmutableList<int> values;
}
```

And that file does **not** have this line:

```cs
using ImmutableObjectGraph;
```

The generated code won't compile because extension method syntax is used that assumes that is present.

Similar to #104