In this way when we order, we always return the same solution order in
case there are weak deps.
The following is optional - it doesn't change the "correctness" of the
solver results: We add an extra edge between deps that
share common dependendencies. This makes the link more stronger and
balances the graph so it doesn't show different results for the same query, as they
could be shuffled as don't have a direct connection.
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.
first step - it is slower in the implementation for now, but all cases
seems to be sorted out.
Drop Flagged() and IsSet() from solver. the solver wont care, and only
the assertion value does matter (exception for uninstall)
