Remove redundant cache_dirs, fix read only cache_dirs issue

Kristian Klausenrequested to merge
github/fork/bartoszek/fix_cache_dirs into master

Created by: bartoszek

Fix #57

  • compartmentalize cache_dirs
    • cache_dirs contains cached directly sourced from $pac_conf file with pacconf CacheDir
    • extra_cache_dirs contains caches form host mirrors and local repositories
    • filter out extra_cache_dirs from cache_dirs preventing using local repository as pacman cache if listed first in pacman.conf
  • instead of inserting caches into each CacheDir= line in $working_dir/etc/pacman.conf on each arch-nspawn call, append new [options] section containing CacheDir=${extra_cache_dirs[*]} to $working_dir/etc/pacman.conf when $working_dir/etc/pacman.conf gets overwritten ( arch-nspawn -C pacman.conf )

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