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



the same logic is [here](https://github.com/fsprojects/Paket/blob/master/src/Paket.Core/Simplifier.fs#L37) - maybe could be reused?