Bug #250020 - When calculating merge order, try to ensure that packages

Bug #250020 - When calculating merge order, try to ensure that packages
listed in DEPEND are updated before whenever possible (even though the
DEPEND may already be satisfied by an installed instance). The changes
to the merge order algorithm should also account for many common cases of
bug #199856, but does not necessarily solve all cases. Whenever possible,
the new algorithm avoids dropping dependencies that are satisfied by installed
packages. Such dependencies are only dropped in a couple of cases:

 * when solving circular dependencies
 * when promoting packages to in the merge list (either
   due an unsatisfied PDEPEND or a portage upgrade)

svn path=/main/trunk/; revision=12612