recover from some repo errors

[Chris Lumens <clumens redhat com>]

Here's two patches that allow us to recover from some errors where repodata
goes missing.  The first one is the bulk of the code but is really boring.
It just reorganizes the repo editing screen I came up with a while back so
that it can be called from more places.  The second one is the interesting one
which lets us use that repo editing screen in case the repodata from your
base repo is missing when we are doing setup.

This is in preparation for the stage2= stuff that's coming later.

- Chris