Remove mirror from default package cache directory cache key again

Remove mirror from default package cache directory cache key again

We added this initially to deal with pacman not having the mirror
in its cache key of repository metadata. The downside of this approach
is that we cannot cache packages across different mirrors. As an
aternative, let's simply not cache repository metadata for pacman in
the package cache directory.

While we're at it, remove the hack we did for zypper to ensure it had
the mirror in its cache key and also don't store its repository metadata
in the package cache directory. The reasoning here is while we can make
sure our own generated repository ids have the hashed mirror in them, we
cannot do so for any repositories added by users, which might end up causing
conflicts.