PrefixedFileProvider with prefix "/" always returns NotFoundFileInfo for every non-root path

[rameel]

var provider = new PrefixedFileProvider("/", innerProvider);

provider.GetFileInfo("/foo/bar.txt");    // returns NotFoundFileInfo
provider.GetDirectoryContents("/foo");   // returns NotFoundDirectoryContents
provider.Watch("/foo/**/*.cs");          // returns NullChangeToken