Support MSBuild Traversal SDK projects fully in the .NET CLI

[baronfel]

Is your feature request related to a problem? Please describe.

While solutions are the traditional way to manage multi-project build and orchestration in the .NET ecosystem due to their IDE support, the Traversal MSBuild SDK is often used by large teams or power users that want more direct control of their repo-wide build experiences. These projects mimic many of the benefits of solutions, while allowing for a more natural way to express build-level dependencies, easier Target definitions, and more 'clean' alignment to traditional MSBuild semantics.

Today, Traversal projects are treated like 'normal' projects by the .NET CLI for non-MSBuild commands. Commands like test, format, reference add, and more attempt to interact just with the Traversal project instead of 'unwrapping' the project to discover its contained project graph - like they do for solutions.

Describe the solution you'd like

The dotnet CLI should natively support traversal projects in all of its commands and experiences. It should provide patterns for contributing teams to correctly consume and work with such projects. It should enable easily creating new traversal projects.

Additional context

Traversal projects are often used by large teams inside Microsoft to make common repo-management tasks easier.

[Youssef1313]

@baronfel I'll need to start doing this for dotnet test very soon.

Do you prefer that when we are given a project, that we just immediately attempt to evaluate it, and look for IsTraversal? Or should we only do this for the .proj extension?

Today, it's dirs.proj by default: https://github.com/microsoft/MSBuildSdks/blob/5bfc41bb8432f695187451ea5bc5b60250433b21/src/Traversal/Sdk/Traversal.props#L14 but can be changed I think.

[baronfel]

@Youssef1313 please don't do anything yet - prioritization discussions are currently happening.

[jackhorton]

@baronfel I saw that dotnet format doesnt like traversal projects over the weekend:

PS> dotnet format dirs.proj
Could not format 'D:\edge\es\4\swarm\dirs.proj'. Format currently supports only C# and Visual Basic projects.

Seems like #42296 should be linked to this as well?

[baronfel]

Thanks @jackhorton - I had been looking for that to no avail.