[Traversal] Support `Platforms` as a First Class Concept

See the first example of https://github.com/dotnet/msbuild/issues/7760#issuecomment-1188463881, TraversalPlusP2PToVcxproj.zip.

This includes a traversal project that simply references all project recursively, and an AnyCPU csproj that references a vcxproj with explicit SetPlatform metadata.
Expected: When the dirs.proj is built, the vcxproj builds once, as its only platform (x64).
Traversal projects should build all platforms of the projects they directly reference.
Actual: The vcxproj builds twice: once (successfully) because of the csproj with the explicit SetPlatform metadata, and once more (failure) because the dirs.proj didn't supply any Platform global property.

The ask: The Traversal SDK should support Platforms as a first class concept, and attempt to build all ProjectReference items based on what's found in Platforms.

Questions:

• What platform should be chosen for each referenced project? dirs.proj's typically don't have a "preferred platform" (correct me if I'm wrong), so I'm not sure how we would "just know" what to build a project as.

cc @AArnott

Start by reproducing the behavior from the linked TraversalPlusP2PToVcxproj.zip example, focusing on dirs.proj, the AnyCPU csproj, and the referenced vcxproj. Read the Traversal SDK's ProjectReference handling and define how Platforms should be discovered and selected so each referenced project is built with the intended platform and without duplicate builds.