With this change the solver during install now considers only the part
of the tree which is required to calculate the solution, it doesn't
consider anymore World() as the search space.
The search space now is narrowed down to the packages that related to
the one which we are considering.
In this subset of changes we are also optimizing the Parallel solver
avoiding an useless loop.
This change boost overall performance on large datasets which don't
necessarly have relations touching the whole tree.
