fix multiple change to csprojfile with some pkg

When using a package that include another dll in his lib folder. And the
other dll exists as a package, then csprojfile are changed multiple
times du to addassemblies save. In that case we have a reload all window
on vs.

Package creating conflicts :
- TweetSharp-Unofficial (embed hammock and newtonsoftjson)

To reproduce the issue, install TweetSharp-Unofficial then newtonsoftjson (or hammock) in project.
You'll see the csproj file wirte change every time the task FixReferences is runned.