dotnet pack mistakenly uses alias for WPF Library projects

[zkat]

When packing a project with a target platform, when the target platform version is not included in the .csproj (but a platform is), the lib files are pulled in using the "alias".

Details about Problem

NuGet product used (NuGet.exe | VS UI | Package Manager Console | dotnet.exe): All of the above

dotnet.exe --version (if appropriate): 5.0.100-rc.1.20454.5

VS version (if appropriate): 16.8.4-pre.4.0

OS version (i.e. win10 v1607 (14393.321)): win10

Worked before? If so, with which NuGet version: no

Detailed repro steps so we can see the same problem

1. mkdir nuget-repro && cd nuget-repro && dotnet new wpflib
2. dotnet pack
3. look in the .nupkg lib directory

Result

We get a facefull of warnings:

❯ dotnet pack
Microsoft (R) Build Engine version 16.8.0-preview-20451-02+51a1071f8 for .NET
Copyright (C) Microsoft Corporation. All rights reserved.

  Determining projects to restore...
  All projects are up-to-date for restore.
  You are using a preview version of .NET. See: https://aka.ms/dotnet-core-preview
  bar -> C:\Users\kmarchan\src\bar\bin\Debug\net5.0-windows\bar.dll
C:\Program Files\dotnet\sdk\5.0.100-rc.1.20454.5\Sdks\NuGet.Build.Tasks.Pack\build\NuGet.Build.Tasks.Pack.targets(198,5): warning NU5128: Some target frameworks declared in the dependencies group of the nuspec and the lib/ref folder do not have exact matches in the other location. Consult the list of actions below: [C:\Users\kmarchan\src\bar\bar.csproj]
C:\Program Files\dotnet\sdk\5.0.100-rc.1.20454.5\Sdks\NuGet.Build.Tasks.Pack\build\NuGet.Build.Tasks.Pack.targets(198,5): warning NU5128: - Add a dependency group for net5.0-windows to the nuspec [C:\Users\kmarchan\src\bar\bar.csproj]
C:\Program Files\dotnet\sdk\5.0.100-rc.1.20454.5\Sdks\NuGet.Build.Tasks.Pack\build\NuGet.Build.Tasks.Pack.targets(198,5): warning NU5128: - Add lib or ref assemblies for the net5.0-windows7.0 target framework [C:\Users\kmarchan\src\bar\bar.csproj]
  Successfully created package 'C:\Users\kmarchan\src\bar\bin\Debug\bar.1.0.0.nupkg'.

...and it's using net5.0-windows instead of net5.0-windows7.0.