Allow Package/Index file with an empty extension

When downloading Index files during an "update" operation there are
often "Index" files without an extension at all. So additionally to
"Index" extensions like: .gz, .lzma, .bz2 also try to download a
file/package without any filename extension.
This does not look nice. We should avoid string parsing as much as we can... Anyways,, you give me the revised patch for my previous comment, and I'll look into this change then.