Just in case same repository is mentioned with different tags.

If n+1 packages depend A, and A depend on B. Add n+1 dependencies
to B. Otherwise if someone conflicts B, B might be left out.

Leaving package unassigned is no longer a non-preferred action,
this fixes the final test case that was failing.

And with --force we might even install that scenario.
Add also some debug checks.

callgrind says it's more overhead than improvement. back jumping
effectively prunes all bad trees. but can be added later if it
becomes needed; due to e.g. provides support.

It is faster to just scan the cache directory for existing packages
at startup than trying to faccessat() them on demand. It also makes
quite a few parts of the code more readable and simpler.

* properly do absolute scoring now, the previous scoring where
  preference could get reduced could have caused incorrect early
  pruning of search tree

* backtracking is now separated from package state, and first
  branching point is the decision if a name is left unassigned
  or if something _has_ to be assigned. this allows multiple future
  search tree optimizations like handling of common dependencies
  early.

* merge common dependency names early to provide deeper forward
  checking.

also, discover late if package is needed or not.

Instead of "skipping" certain packages, we include them as-if required,
and at expansion time we decide if they actually need to be considered
for installation. This cleans up the expansion main loop a little bit
and makes the code work together better.

* cleaned up little bit on the internal state machine
* the decision applying mechanism now aborts early to avoid work
  if we are approaching bad solution candidate
* package availability checking is now done on-demand; which
  could still be improved

name state could get overwritten later, so we can't use that when
generating the changeset.

* solver internally calculates now using tags; not repository masks
* installeddb now contains the tag name where the package came from
  -> we can now handle upgrades properly
* the pinning is still a preference, and not strictly enforced;
  versioned dependencies may overrule preference

And implement --directory-permissions for fix-applet to reset all
directory uid, gid and modes.

Only show a warning if we have a non-existing repository or bad
repository signature but don't abort.

This is not to break compat with behaviour in previous releases.

The 'r' variable is overwritten and no longer holds the repository
number.

Also clean up handling of signature failures for index files.

Forgot to reset per-name penalty when it got locked by apply_decision.
This also fine tunes compare_package_preference() to always prefer
packages specified on command line speeding up calculation certain
complicated solutions.

 * upgrade needs explicit check so we don't try self-upgrade
   (which would print additional messages on screen)
 * add can fix problems, so check against the new world
 * merge the code in few places

regression from upgrade to the new solver system.