Exclude resharper files
Drastically reduce memory usage when indexing packages

Previously, a folder with a thousand 4mb packages would consume over 600MB of memory during indexing. That's because the implementation relied on ZipPackage, which likes to load everything into memory. Then, it loads all file bytes into memory again just to calculate a file hash, which can just as easily be calculated from a stream (I suspect this came from NuGet.Server code). I'm bypassing this part of NuGet by loading the metadata from the ZIP manually via System.IO.Packaging.