Selective update should only update dependent packages

@agross showed in https://github.com/fsprojects/Paket/issues/400 that we fix to few packages in selective updates.

Before: we fixed all direct dependencies from the paket.dependencies file and ignore indirect dependencies
Now: we fix all packages that are not dependencies of the package in question



@forki You could refactor it further to use an accumulator parameter, than we can call
`addPackage package (HashSet<_>())` and remove this `usedPackages`.