mkosi: stop using deepcopy when parsing arguments
mkosi: stop using deepcopy when parsing arguments

The code was modifing the passed argv array internally. And in fact,
parse_args_distribution_group() only worked correctly if
parse_args_file_group() was previously used to modify the argv array. Let's
drop all this, and never modify the input arguments. In the few cases where
it's appropriate, a copy is made internally.

Also shorten names of internal variables. defaults_file vs. default_file vs.
all_defaults_files was only making the code harder to read.

systemd/mkosi,systemd/mkosi