Include localPath in deps.json for runtime assets

[elinor-fung]

Include localPath property in deps.json for runtime assets

• Determine the local path based on DestinationSubDirectory metadata + file name
• Use RuntimeFile and ResourceAssembly constructors that take a local path

The tests in GivenThatWeWantToBuildANetCoreApp validate the generated .deps.json and the current behaviour when running the produced output. They also show adding a target in the test project to put dependencies in a custom subdirectory - until the runtime change to actually use the localPath property (dotnet/runtime#118297) gets in and over to the sdk repo, putting dependencies in custom subdirectories will fail at run time.

Resolves #48406

cc @dotnet/appmodel @AaronRobinsonMSFT

[Copilot]

Pull Request Overview

This PR adds support for including localPath properties in deps.json files for runtime assets, enabling dependencies to be placed in custom subdirectories while maintaining proper runtime resolution.

• Extends dependency context generation to include local paths based on DestinationSubDirectory metadata
• Updates RuntimeFile and ResourceAssembly constructors to accept local path parameters
• Adds comprehensive tests for both package dependencies and project references with custom subdirectories

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
test/Microsoft.NET.Build.Tests/GivenThatWeWantToBuildANetCoreApp.cs	Adds new test methods to validate localPath generation and runtime behavior with custom subdirectories
src/Tasks/Microsoft.NET.Build.Tasks/ReferenceInfo.cs	Extends ReferenceInfo class to track DestinationSubPath for references
src/Tasks/Microsoft.NET.Build.Tasks/GenerateDepsFile.cs	Updates deps file generation to include local path information from task items
src/Tasks/Microsoft.NET.Build.Tasks/DependencyContextBuilder.cs	Modifies dependency context building to pass local path to RuntimeFile and ResourceAssembly constructors
src/Tasks/Microsoft.NET.Build.Tasks.UnitTests/simple.dependencies.deps.json	Updates test fixture to include expected localPath properties in deps.json
src/Tasks/Microsoft.NET.Build.Tasks.UnitTests/GivenADependencyContextBuilder.cs	Adds unit tests for local path inclusion in dependency context building

[Copilot]

Changing UserRuntimeAssemblies from string[] to ITaskItem[] is a breaking change that could affect consumers of this task. Consider maintaining backward compatibility by providing an overload or conversion mechanism.

Suggested change

	// Backward compatibility for string[] property
	[Obsolete("Use UserRuntimeAssemblies as ITaskItem[] instead. This property is for backward compatibility.")]
	public string[] UserRuntimeAssembliesAsString
	{
	get => UserRuntimeAssemblies?.Select(item => item.ItemSpec).ToArray();
	set => UserRuntimeAssemblies = value?.Select(s => new TaskItem(s)).ToArray();
	}

[elinor-fung]

I don't really know what the best thing to do here is. For usage via MSBuild targets, I think the passed strings convert to ITaskItem[] ("value1;value2" is two items with the itemspec set those values and no additional metadata).

Is external usage of this task via C# expected?