.pri files are not automatically referenced in .NET core projects

### Steps to reproduce

I've attached two .sln files and some binlogs
https://microsoft-my.sharepoint.com/:f:/p/stevenki/EixvC6_FBmRAqUopltGOtA0B5_f46g0zwGsgcfttI25jhg?e=a4sTuS

You can open each .sln in Visual Studio and F5 them. For App6, you'll need to make sure the `WapProjTemplate1` project is the startup project.

### Expected behavior

### Actual behavior

### Environment data
`msbuild /version` output:

c:\Users\stevenki\source\repos>msbuild /version
Microsoft (R) Build Engine version 16.7.0-preview-20228-06+9571f7584 for .NET Framework
Copyright (C) Microsoft Corporation. All rights reserved.

16.7.0.22806

OS info: Windows Insider build 20124

If applicable, version of the tool that invokes MSBuild (Visual Studio, dotnet CLI, etc):
VS: 16.7.0 Preview 1.0 [30104.155.master]
Dotnet CLI: N/A (apps can't build w/ dotnet cli)

### Cause
What I found in my investigation, is that the `ResolveAssemblyReferences` task is supposed to return the .pri in the `_ReferenceRelatedPaths` item group. However, even though the .pri extension is passed in as an allowed extension, that isn't being respected. From there, it's a chain reaction that's basically causing the .pri file to not be merged into the apps .pri, which results in xaml files not being found at runtime