Publish PowerShell NuGet packages for Linux/OSX platforms

[daxian-dbw]

PowerShell Core builds separately for Windows and Unix, so the assemblies are different between Windows and Unix platforms. All PowerShell Core packages we published so far contain windows assemblies only.

The ideal solution is to have one package that contains assemblies for both Windows and Unix platforms. But dotnet pack doesn't seem to support this.

This is blocking hosting powershell in .NET Core applications on Linux/OSX.
The discussion of this issue originates from #3409

[mirichmo]

We need to publish the Windows version of the packages to NuGet as well.

Note: This issue seems very similar to #1152

[lzybkr]

I think we want a single set of reference assemblies for portability. If we have platform specific APIs, then hopefully there is a way to refactor those into another assembly if need be.

[daxian-dbw]

Take System.Management.Automation as an example, I think we will eventually end up with 2 set of NuGet packages:

1. A NuGet package that only contains the reference assembly. This will be the PowerShell Standard reference assembly, which exposes an API surface that is supported by both Windows PowerShell and PowerShell Core. But the API surface will a subset of the actual publish APIs exposed in Windows PowerShell or PowerShell Core.

   • This reference assembly is an abstraction that doesn't have any runtime implementation that exactly maps to it, so there won't be any runtime DLLs in this NuGet package.
   • This NuGet package is the friend of powershell binary module authors. Binary modules targeting this NuGet package and netstandard2.0 will theoretically work on both Windows PowerShell and PowerShell Core.

2. A NuGet package that is specific for PowerShell Core. It contains the reference assembly that exposes all public APIs from the latest System.Management.Automation.dll in PowerShell Core, as well as the runtime assemblies for both Win and Unix platforms, including the native dynamic library libpsl.so that powershell depends on in Unix plats.

   • This NuGet package is supposed to be used by any netcoreapp2.0 application that needs to host PowerShell Core.