AssignCulture task incorrectly categorizes some EmbeddedResources as culture-specific

### Steps to reproduce

Create a class library with a file `Index.Id.cshtml` marked as embedded resource.
Build.

Project file
```xml

netcoreapp2.0

PreserveNewest

```

### Expected behavior
Should be part of the embedded resource of the assembly

### Actual behavior
A resource assembly for the `Id` culture is created and this file added to it.

My understanding is that when the `AssignCulture` task runs, it finds `Id` to be a valid culture. I confirmed it by looking at `msbuild.log`.

I could suggest for this task to look into a group properties to skip the files. So I could at least mark the files as non resx.