Personally, I'm happy to require people to install the SDK. There are benefits to using the SDK (versioning is one - if you build with the SDK, it will work with 8.0.0, 8.0.1 and 8.0.2. If you build against your program files dir, you can make your plugin require 8.0.2 and cause issues for other users), and by building against ReSharper rather than the SDK, you've just substituted one dependency for another - you still require them to have ReSharper installed. Basically, I don't think it's too much to ask to install the SDK.

Things are easier with the nuget packages - package restore solves that issue. There is an 8.0 SDK on nuget, too, although if you put two projects in the same folder, they can't currently have separate packages.config files (this is fixed in nuget 2.8), so you pretty much need an installed SDK for 8.0 right now.