MSBuild::NormalizePath does not correctly canocalize paths containing back-slashes on xplat

### Steps to reproduce

Project file
```xml

$([MSBuild]::NormalizePath('$(MSBuildThisFileDirectory)..\test'))
$([MSBuild]::NormalizePath('$(MSBuildThisFileDirectory)../test'))

```

On Windows, the result is:

```
D:\test D:\test
```

On Ubuntu, this is what I get:

```
/mnt/d/utemp/../test /mnt/d/test
```

This is not a problem since the File I/O APIs would resolve either paths. However, in this particular I was trying to remove an item from an ItemGroup using it's canonicalized path.