Change the problem data arrangement for dependency problems

Change the problem data arrangement for dependency problems
- pkgNEVR in problems is now always the NEVR of the transaction element
  triggering the problem, and altNEVR is the other affected package,
  dependency string is stored in the problem string attribute
- no user visible changed, except for somebody crazy enough to try to
  do something other than print the problem message strings
- we wouldn't really need to strdup() the pkgNEVR in problems now,
  but leaving that alone for the moment...