package_rpm/deb/ipk: Error if we don't find packages when creating the package index
If for whatever reason the package directory is empty of packages, it
makes sense to error early rather than later in what become much
more obtuse errors. This adds in a sanity check to each of the packaging
backends. It also removes the duplicate createrepo call since the
core index creation function now uses this directly after the switch
to smart.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
