Use the official provider list to detect duplicates

Use the official provider list to detect duplicates

Instead of automatically adding packages from the official binary
repositories to the package blacklist, use the official provider list to
prevent users from uploading duplicates.

This does not only result in reduced disk usage but also has a nice
visible side effect. The error messages printed by the update hook now
look like

    error: package already provided by [community]: powerline-fonts

instead of

    error: package is blacklisted: powerline-fonts

which was confusing to most end users.

Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>