Convert non-seekable PackagePart streams to seekable (#1311)

Create extension methods for PackagePart that allow us to acquire a seekable stream in the case where the stream returned from PackagePart.GetStream is not already seekable.  Switch instances where we call PackagePart.GetStream to use this in order to work-around the fact that a read-only PackagePart stream will use DeflateStream, which is not seekable in .NET Core.

Author: rladuca