solver3: Order obsolete choices last

This has two aspects:

1. For a dependency A | B | C we order the obsolete packages last,
   that is, if A is obsolete, this gets reordered to B | C | A,
   such that we try to pick non-obsolete packages first to ease
   upgrade calculation.

2. When comparing two dependencies, we order dependencies into three
   groups: First we satisfy dependencies mentioning only non-installed
   (NEW) packages, then we satisfy "normal" dependencies, and finally
   we satisfy any dependencies mentioning obsolete packages.

   This means for example if you have obsolete libfoo1 and a new
   libfoo1t64, that we will see Depends: libfoo1t64 before any
   Depends: libfoo1 (which may expand to libfoo1 | libfoo1t64),
   so we effectively will have selected "replacement" packages
   this way already before getting to older packages where we
   would have to choose between the obsolete package and its
   replacement.

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