Reduce allocations in VirtualFileInfo.Name

[zivkan]

VisualFileInfo is an implementation of IFileInfo used with Microsoft.Extensions.FileSystemGlobbing. It's used when matching files in the package to files listed in the contentFiles section of package nuspec files.

The property is getting the filename from a path, but it does so with PhysicalPath.Split('/').LastOrDefault();. Both Split and LastOrDefault cause allocations, plus the result is not cached and debugging shows the same instance can have the Name getter called multiple times.

This code is used when generating the assets file when restoring a PackageReference project that uses a package that has contentFiles/* assets.