Refuse to install obsoleted packages (RhBug:486565)

Refuse to install obsoleted packages (RhBug:486565)
- Packages which are obsoleted by some installed package now behave
  much like conflicts, ie install is refused unless --nodeps is used
- Unlike conflicts, obsoletes are only tested against installed package
  names and not provides. Otherwise it would be impossible to create
  compat-packages which provide something that is obsoleted.
- Only check against installed obsoletes, as any obsoletes in the
  transaction could and should be handled automatically similarly to
  foo-1.0 and foo-2.0 in the same transaction set resulting only in foo-2.0
  getting installed.