repo: Fix file structure

repo: Fix file structure

Because it is possible to have multiple builds with the same nevra in
the same repository, we need to store packages in a subdirectory to be
able to uniquely identify them. That is also helpful when we want to
avoid downloading a package when expecting a different UUID.

This patch changes that packages will now be put into some subdirectory
structure so multiple archives with the same name can coexist.

The downloader and caching mechanism for repositories had to be tweaked
to honour the subdirectory structure.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>