Handle includepkgs and exlude entries in repositories.

Sometimes we want to blacklist some packages from a repository.  Or, a more common case, we want to disable a repository entirely, but yet install one or two packages from it.

An easy example is rpmforge: its contents often conflict with EPEL, but yet there are a few useful packages that we may need in production.  We want to get *only* those packages from rpmforge, and prevent any further conflicts.