downstream: Allow unmarking a package as downstream-only
models: New 'upstream_pkgname' attribute for packages

This will be used to indicate that a package in the current distro has
in fact been renamed from its upstream.
views: Allow marking a package as renamed
sync: Rework the get_upstream_evr function

The code was a bit messy, due to the very long variables and indexing of
a dictionary of dictionaries...
sync: s/pkg.name/pkgname

Next commit will actually depend on that new variable.
sync: Properly handle renamed packages

They have been marked as being renamed from upstream, so when querying
for the upstream EVR, we should do so using the proper name of the
package, as it exists upstream.
status: Display the upstream name of renamed packages
problems: Allow marking a package as renamed
overview: Add a entry for the renamed packages

Now that these aren't problems any more, they are displayed along
regular up-to-date / out-of-date packages.

Nevertheless, it can be interesting to view them all in the same place.
status: Report status of renamed packages
renamed: Allow changing the upstream name of a renamed package

This also allows unmarking a package as renamed, by simply setting the
new name to an empty string.