Handle the Aliases compile item metadata for PackageReference

[nkolev92]

Build tasks side of NuGet/Home#4989.

Design at https://github.com/NuGet/Home/blob/dev/designs/PackageReference-Extern-Alias.md.

As part of LockFileItem for compile items, the SDK needs to pass through the aliases property to the compiler.

Equivalent to dotnet/sdk#10947.