"kpm pack" identifies source files outside of project root and packs them

parent #570 

The original implementation only packs source files inside project root dir. In this PR, if a source file is out of project root dir but in the solution dir, we pack it as well.
Don't use this, we have a helper in the runtime. PathUtility.GetRelativePath (https://github.com/aspnet/KRuntime/blob/dev/src/Microsoft.Framework.Runtime/NuGet/Utility/PathUtility.cs#L63)