dotnet/sdk#1458 depends on this

To include PDB to CopyLocal, SDK need NuGet.ProjectModel to categorize PDB and XML file to an ideally different category like "debug symbol" or "related file".

So SDK can get all the symbol files by something like var symbolPath = lockFileTargetLibrary.debugSymbols , just like CompileTimeAssemblies, RuntimeAssemblies today

https://github.com/NuGet/NuGet.Client/blob/efae789c75899ca0bdd063f34a4b73cd41510d56/src/NuGet.Core/NuGet.ProjectModel/LockFile/LockFileTargetLibrary.cs#L27

• Add a spec of "consuming pdbs from packages in PackageReference" #11350
• Check existing extensions besides assemblies in packages on nuget.org #11351
• Implement "consuming pdbs from packages in PackageReference" #11352