Description of problem:
"dnf update" gets confused by Obsoletes in some packages and attempts to replace a package by itself. Eventually it hits a traceback due to an attempt to install a package from a repo called "@System".

Version-Release number of selected component (if applicable):
dnf-0.4.4-1.fc20.noarch
hawkey-0.4.3-1.fc20.x86_64
librepo-1.2.1-3.bc2c8a5.fc20.x86_64
rpm-4.11.1-7.fc20.x86_64

How reproducible:
always

Steps to Reproduce:
1. Start with having the following packages installed:
     iasl-20120913-7.fc19.x86_64
     acpica-tools-20130823-2.fc20.x86_64

   Note that this build of acpica-tools has "Obsoletes: iasl < 20120913-7",
   which is not high enough to obsolete the currently installed iasl.

   Also note that a new build of acpica-tools is available in the repositories:
   acpica-tools-20130823-5.fc20. This one has bumped the obsoletes to
   "Obsoletes: iasl < 20120913-8", which should get rid of my currently
   installed iasl.
2. dnf update

Actual results:
Resolving dependencies
--> Starting dependency resolution
---> Package iasl.x86_64 20120913-7.fc19 will be obsoleted
---> Package acpica-tools.x86_64 20130823-2.fc20 will be upgraded
---> Package acpica-tools.x86_64 20130823-2.fc20 will be an upgrade
--> Finished dependency resolution


Dependencies resolved.

===============================================================================
 Package       Arch            Version                  Repository        Size
===============================================================================
Upgrading:
 iasl          x86_64          20120913-7.fc19          @System          1.3 M
     replacing  iasl.x86_64 20120913-7.fc19

Transaction Summary
===============================================================================
Upgrade  1 Package

Total download size: 1.3 M
Is this ok [y/N]: y
Downloading Packages:
Traceback (most recent call last):
  File "/bin/dnf", line 35, in <module>
    main.user_main(sys.argv[1:], exit_code=True)
  File "/usr/lib/python2.7/site-packages/dnf/cli/main.py", line 272, in user_main
    errcode = main(args)
  File "/usr/lib/python2.7/site-packages/dnf/cli/main.py", line 64, in main
    return _main(base, args)
  File "/usr/lib/python2.7/site-packages/dnf/cli/main.py", line 187, in _main
    return_code, resultmsgs = base.do_transaction()
  File "/usr/lib/python2.7/site-packages/dnf/cli/cli.py", line 185, in do_transaction
    self.download_callback_total_cb)
  File "/usr/lib/python2.7/site-packages/dnf/yum/base.py", line 1024, in download_packages
    remote_pkgs = filter(lambda po: not (po.from_cmdline or po.repo.local), pkglist)
  File "/usr/lib/python2.7/site-packages/dnf/yum/base.py", line 1024, in <lambda>
    remote_pkgs = filter(lambda po: not (po.from_cmdline or po.repo.local), pkglist)
  File "/usr/lib/python2.7/site-packages/dnf/package.py", line 109, in repo
    return self.yumbase.repos[self.reponame]
KeyError: '@System'


Expected results:
"yum update" gives more sensible results:

Resolving Dependencies
--> Running transaction check
---> Package acpica-tools.x86_64 0:20130823-2.fc20 will be updated
---> Package acpica-tools.x86_64 0:20130823-5.fc20 will be obsoleting
---> Package iasl.x86_64 0:20120913-7.fc19 will be obsoleted
--> Finished Dependency Resolution
--> Finding unneeded leftover dependencies
Found and removing 0 unneeded dependencies

Dependencies Resolved

===============================================================================
 Package           Arch        Version              Repository            Size
===============================================================================
Installing:
 acpica-tools      x86_64      20130823-5.fc20      updates-testing      683 k
     replacing  iasl.x86_64 20120913-7.fc19

(and yum would go on to update successfully)