Add support for Microsoft.Build.Traversal projects

[just-ero]

This one might be a bit of a reach, but I've heard from many people that they would like traversal projects to act as solutions. Personally, it's the ease with which projects can be included. Traversal projects could make use of the familiar MSBuild properties, items, targets, and tasks known from project files.

<Project Sdk="Microsoft.Build.Traversal/4.1.0">

  <ItemGroup>
    <ProjectReference Include="src/*/*.csproj" />
  </ItemGroup>

</Project>

I'm curious about people's alternative ideas, since I'm not optimistic that this can be supported. MSBuild projects are a lot more complicated, and using one as a solution would probably produce unexpected behavior.

---

I would still like to open the discussion for some format that's closer to MSBuild project files, so let me know if you have any ideas on how to perhaps use the MSBuild architecture to build a solution format that integrates with MSBuild. A possible format I've imagined is the following:

<Project Sdk="Microsoft.Build.Solution">

  <ItemGroup>
    <Configuration Include="CustomConfiguration" Platform="Any CPU" />
  </ItemGroup>

  <ItemGroup>
    <Project Include="src/*/*.csproj"
             Folder="src" />
    <Project Include="test/*/*.csproj"
             Folder="test" />

    <Project Update="test/*/*.csproj"
             Condition="'$(Configuration)' == 'Release'"
             NoBuild="true" />
  </ItemGroup>

</Project>

[richardstanton]

This is currently out of scope, but the current design attempts to separate configuration data from project data to leave the door open for multi-file formats.

[just-ero]

I realized a couple days after opening this issue that supporting such a format seems far out of scope for at least this repo. I'm hoping that solution support can be moved entirely into MSBuild. It appears to be a very fitting tool for the job and would make expansion of the format easier, I imagine, since MSBuild already has very well-known and established syntax and more complete features like file globbing and resolving full paths.