nuget improvements

• Now supports specifying Version and Repository in AdditionalOpenTapPackage items. (example below)
• Fixes issue where MSBuild.dll could not be found, or it could not find tap.exe (by not depending on the MSBuildTask defined in that dll at all)
• Don't assume that users want to debug using Editor.exe (users might not have Editor.exe)
• More robust way of triggering the CopyOpenTapPayloadFiles target

Example *.csproj file:

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

  <PropertyGroup>
    <OutputType>Library</OutputType>
    <TargetFramework>netstandard2.0</TargetFramework>
    <OpenTapPackageDefinitionPath>package.xml</OpenTapPackageDefinitionPath>
  </PropertyGroup>

  <ItemGroup>
    <AdditionalOpenTapPackage Include="Developer's System CE" Version="9.2.2" Repository="package.opentap.io"/>
  </ItemGroup>

  <ItemGroup>
    <PackageReference Include="OpenTAP" Version="9.2.2" />
  </ItemGroup>

</Project>