Solver algorithms:
------------------

- the solver allows arch changes from/to noarch without complaining.
  good or bad?

- vendow changes are allowed from unknown to known. libzypp doesn't
  allow that. Feature?

- arch changes are only considered if the name stays the same, so
  a rename can install a different architecture.

- same with vendor.

- should prune_best_version_arch consider the arch or the vendor this
  is about an installed package?

- we disable conflicting rules when searching for a suggestion, should
  we first extend them with arch changes/vendor changes/downgrades?
  In what order?

- distupdate sets 'allowuninstall'. Should it only allow uninstall
  for packages that do not have an update in the repositories?

- splitprovides don't work at all.

- repo priorities should influence the order in which package rules
  are fulfilled.

- weak systemrules should be created for all installed packages, so
  that they are available when the erase rule is disabled if a
  suggestion is calculated.

- should we implement weak systemrules as weak rules?

- prune_best_version_arch has a n^2 loop when looking at obsoletes.
  Should we create an obsoletes index to speed that up?



Code cleanup:
-------------

- move whatprovides/whatprovidesdata from pool to solver?
  good: can implement installed(), though I don't know if we need
        it.
  bad: need to allocate solver if we just want to look up a provides.
       can't re-use whatprovides for multiple solver runs with different
       solvers.

- implement unified idarray in pool, so that repos don't have to
  come with their own idarrays? Would need memory for hash, but
  unification would be a nice thing. Would also fix solver memory
  leak with learned rules. Needs some hard numbers abour memory usage.

- make bitmap macros inline functions?

- rename ID Macros to ID_, e.g. ARCH_NOARCH to ID_ARCH_NOARCH?

- support different streams than FILE in repo_ functions? Is it
  enough that glibc supports FILE overloading, see the undocumented
  fopencookie()?