Fabian Ebner [Wed, 30 Jun 2021 10:20:16 +0000 (12:20 +0200)]
add get_cached_origin method and an initial config module
Allows obtaining the 'Origin' property from the cached InRelease file.
Used the once_cell crate for the config module, because it is already
used in proxmox-backup and seemed to be the right fit here.
For now, the config module is just used to be able to override the
path for the test environment, but those are actual APT config
variables, and in the future, it can be extended and used to actually
parse the apt.conf(.d/*) on the system.