Incorrect code generation when declaring an observable property with a source-generated type

### Describe the bug

Declaring an observable property with a source-generated type fails at compile-time.

Source generators should have access to other source-generated code.

### Steps to reproduce

```text
1. Create a project
2. Add CsWin32
3. Add CommunityToolkit.MVVM
4. Register `WindowFromPoint` in the `NativeMethods.txt` file
5. Add an observable property of type `HWND`
6. `The type or namespace name 'HWND' could not be found ...\SpySharp.Views.WPF.MainWindow.g.cs`
```

### Expected behavior

The project should compile as it does with non-source-generated types.

### Screenshots


### IDE and version

VS 2022 Preview

### IDE version

VisualStudio.17.Preview/17.7.0-pre.2.0+33808.371

### Nuget packages

- [ ] CommunityToolkit.Common
- [ ] CommunityToolkit.Diagnostics
- [ ] CommunityToolkit.HighPerformance
- [X] CommunityToolkit.Mvvm (aka MVVM Toolkit)

### Nuget package version(s)

8.2.1

### Additional context

_No response_

### Help us help you

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