Modernize torchsharp infrastructure

[ericstj]

In looking at Torchsharp's build I had a few thoughts for improvement.

• Evaluate using arcade and provided build templates.
• Remove as much as possible from the YML and have it be part of the local build scripts so that it's easier to repro things that are hit by the official build.
• Remove use of the nupkgproj and instead pack from the CProj projects where possible
• For the remaining nupkgproj have them use the Microsf.Build.NoTargets SDK to avoid C# specific targets.
• Add all projects to the SLN, and control the pack behavior with IsPackable conditions rather than conditions in a project

  TorchSharp/pkg/pack.proj

  Line 4 in 077780d

  <PackProject Include="**\libtorch-cpu.nupkgproj" Condition="'$(IncludeLibTorchCpuPackages)' == 'true'" />

• Adopt package readme's following a similar convention to dotnet\runtime.