How to add p2p dependencies in a nuget package when using dotnet pack

[johnkors]

nuget.exe has a switch to solve this common use case -IncludeReferencedProjects, but dotnet pack does not.

• "donet pack" is not including project references dotnet/sdk#6688 (comment)
• Feature : Allow project reference DLLs to be added to the parent nupkg for pack target like IncludeReferencedProjects in nuget.exe NuGet/Home#3891

With this change, the nuget contents will be:

Contents:
  - File:    _rels/.rels
  - File:    [Content_Types].xml
  - File:    icon.512x512.png
  - File:    lib/net5.0/SomeClassLib.dll
  - File:    lib/net5.0/SomePeerDependency.dll 👈
  - File:    lib/net6.0/SomeClassLib.dll
  - File:    lib/net6.0/SomePeerDependency.dll 👈
  - File:    package/services/metadata/core-properties/f9524ad71fec4ad3bed3d935dd3671f3.psmdcp
  - File:    README.md
  - File:    SomeClassLib.nuspec