Introduce a new "cache" subdirectory in the pmbootstrap workdir, all the
cache and config bits go in here, anything that needs to be accessible
from inside a chroot. The whole dir is then bind-mounted into the chroot
as /cache with appropriate symlinks.
This dir is in the config as config.cache.
In addition, all the cache_* and other config dirs are renamed to
be closer to the names of the equivalent dirs in the chroot (e.g.
abuild-config) and to avoid redundant naming since they are now under a
"cache" dir.
Signed-off-by: Casey Connolly <kcxt@postmarketos.org>