Switch to use of MSBuild to create nuget packages

[CharliePoole]

The following packages use a nuspec and will be converted...

• NUnit.Engine.Api
• NUnit.Agent.Core
• NUnit.Engine
• NUnit.ConsoleRunner
• NUnit.ConsoleRunner.NetCore
• NUnit.Console

Chocolatey package nunit-console-runner will continue to use a nuspec file for the time being.

[CharliePoole]

In the end, three packages continue to use a .nuspec file for different reasons...

• NUnit.ConsoleRunner needs one to specify the default bundled agents.
• NUnit.Console is a metapackage
• Chocolatey package nunit-console-runner has special requirements not handled by MSBuild.

[CharliePoole]

This issue has been resolved in version 4.0.0-beta.2

The release is available on:
GitHub.
NuGet packages are also available NuGet.org and
Chocolatey Packages may be found at Chocolatey.org