Support dynamically skipping projects

[jeffkl]

At the moment, users have to manually implement the ability to condition out project references in Traversal projects:

<Project Sdk="Microsoft.Build.Traversal">
  <ItemGroup>
    <ProjectReference Include="src\Common\Common.csproj" />
    <ProjectReference Include="src\App\App.csproj" />
    <ProjectReference Include="src\WebApplication\WebApplication.csproj" Condition="'$(DoNotBuildWebApp)' == 'true'" />
  </ItemGroup>
</Project>

It would be very handy for Microsoft.Build.Traversal to have a built-in mechanism for skipping projects. It would also be great if Microsoft.Build.Traversal could extend Visual Studio solution file builds to dynamically skip projects. This would allow a repo owner to skip projects when a user uses dotnet build instead of MSBuild.exe since not all project types are buildable with dotnet build.

[vikukush]

This new feature breaks the build because GetProjectsToSkip runs before Restore and does not ignore missing imports from packages, causing errors like
error MSB4019: The imported project "C:\msbuild\CompileModule.proj" was not found. Confirm that the expression in the Import declaration "\msbuild\CompileModule.proj" is correct, and that the file exists on disk.
4.0.0 breaks the build, I'm rolling back to 3.1.6

[vikukush]

@jeffkl could you please create a unit test about this? This seems to be a rather standard scenario that got broken; if it's not automatically tested, then it's an omission.

[jeffkl]

@vikukush you can skip updating to the latest version if its broken for you or you can disable the feature by setting the MSBuild property SkipProjects to false for now. I'll work on a fix in about two weeks when I'm back from vacation.