WPF content files don't work with single-file

### Description

Create a WPF application that uses [ContentFiles](https://docs.microsoft.com/en-us/dotnet/desktop/wpf/app-development/pack-uris-in-wpf?view=netframeworkdesktop-4.8#content-file-pack-uris)

When run outside single file this works and can load content.

When run under singlefile this fails because this throws:
https://github.com/dotnet/wpf/blob/eaece0ac93dd660dedcb33d9d908ea7485a8a636/src/Microsoft.DotNet.Wpf/src/PresentationFramework/MS/Internal/AppModel/ContentFilePart.cs#L113

Resulting in null return value which later throws here:
https://github.com/dotnet/wpf/blob/eaece0ac93dd660dedcb33d9d908ea7485a8a636/src/Microsoft.DotNet.Wpf/src/PresentationFramework/MS/Internal/AppModel/ContentFilePart.cs#L76

Most likely this ContentFilePart needs to "know" about single-file and be able to fetch it's content file.

```
System.ArgumentNullException: Value cannot be null. (Parameter 'baseUri')
at System.Uri..ctor(Uri baseUri, String relativeUri)
at MS.Internal.AppModel.ContentFilePart.GetStreamCore(FileMode mode, FileAccess access)
at System.IO.Packaging.PackagePart.GetStream(FileMode mode, FileAccess access)
at MS.Internal.IO.Packaging.PackagePartExtensions.GetSeekableStream(PackagePart packPart, FileMode mode, FileAccess access)
at MS.Internal.IO.Packaging.PackagePartExtensions.GetSeekableStream(PackagePart packPart)
at System.Windows.Application.GetContentStream(Uri uriContent)
at wpf.MainWindow..ctor()
```

### Configuration

net5.0-windows, 5.0.0-rc.2.20464.8, publishSingleFile, win-x64

### Regression?

No, single file WPF didn't work until RC2

### Other information

Repro:
[wpf.zip](https://github.com/dotnet/runtime/files/5228736/wpf.zip)

To repro run `dotnet publish -p:PublishSingleFile=true -r:win-x64 -f:net5.0-windows`

Compare execution of normal vs single file app.