Allow referencing virtual projects

[jjonescz]

Part of dotnet/sdk#53478.

[Copilot]

Pull request overview

Enables ProjectReference to work with virtual (in-memory) projects when _BuildNonexistentProjectsByDefault is enabled, supporting SDK scenarios like file-based apps that synthesize projects into the ProjectRootElementCache.

Changes:

• Update _SplitProjectReferencesByFileExistence to treat all ProjectReference items as “existent” when _BuildNonexistentProjectsByDefault=true.
• Add unit tests validating both the split behavior and end-to-end ResolveProjectReferences behavior with in-memory projects.
• Extend the spec to document ProjectReference behavior and virtual project limitations (notably out-of-proc node isolation).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
src/Tasks/Microsoft.Common.CurrentVersion.targets	Bypasses Exists() classification for project references when building nonexistent projects by default, enabling virtual references.
src/Build.UnitTests/BackEnd/MSBuild_Tests.cs	Adds coverage for virtual ProjectReference behavior (split + end-to-end build).
documentation/specs/build-nonexistent-projects-by-default.md	Documents new ProjectReference behavior and clarifies constraints for virtual projects.

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[jjonescz]

@dotnet/msbuild for a review of this simple change that would allow us to implement #:ref support for file-based apps; thanks!