Add support for easy DesignTime Assembly Packing

[KevinRansom]

Type providers may be split into two portions a runtime helpers library and a design time library to run in the compiler/fcs/fsi etc ...

This PR makes dotnet pack easier.

Example projects:

This Example consists of a runtime project a design time project
The design time project is a project reference it is tagged with:

      <IsFSharpDesignTimeProvider>true</IsFSharpDesignTimeProvider>
      <PrivateAssets>all</PrivateAssets>

Private Assets, stops the build and nuget from making the DesignTime assembly a dependency of the package.

The package layout conforms to : #3864

However, nuget doesn't make make specific target runtimes subdirectories of the TargetFramework, so neither does this. nugget uses the runtime branch for runtime specific probing.

I propose revising the current probing logic to stop probing for x86 and x64.

Additionally:

• DesignTime Assemblies will cover extensions other than TypeProviders moving forward.
• 'typeproviders' causes nuget packaging warnings

The default here is to place designtime assemblies below tool. To place them below typeproviders
use:

<FSharpToolsDirectory>typeproviders</<FSharpToolsDirectory>

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFrameworks>net461;netstandard2.0</TargetFrameworks>
  </PropertyGroup>

  <ItemGroup>
    <Compile Include="BasicProvider.Runtime.fs" />
  </ItemGroup>

  <ItemGroup>
    <ProjectReference Include="..\BasicProvider.DesignTime\BasicProvider.DesignTime.fsproj">
      <IsFSharpDesignTimeProvider>true</IsFSharpDesignTimeProvider>
      <PrivateAssets>all</PrivateAssets>
    </ProjectReference>
  </ItemGroup>

</Project>