Drop "first assignment wins" logic

Drop "first assignment wins" logic

From experience in the systemd repository's usage of presets, we've
learned that we want to have fixed values for certain settings for
presets that cannot be overridden by either the CLI options or by
global config. Good examples are that the Format= of a base image
should always be "directory" and the Format= of an initrd image
should always be "cpio" or that Bootable= should always be "no" for
both the base image and the initrd image and their list of packages
should not be affected by any packages specified on the CLI.

The issue is that with "first assignment wins" logic, we need to add
an explicit "override" mechanism which almost all settings in these
presets would then use to make sure they can't be changed by CLI
options. This seems rather backwards, and is a good indication that
any settings configured in config should not be overridden by settings
set on the CLI.

Even disregarding usage of presets, any existing mkosi config will almost
certainly not be written to expect arbitrary changes in the config due
to options set on the CLI.

Also, it's currently not entirely trivial to set default values for
presets from the global config, because any values set in the global
config cannot be overridden anymore by presets. By not doing "first
assignment wins" logic, this becomes trivial as the global config can
simply set a default value and it can be overridden by presets.

Of course by removing "first assignment wins" logic, we do introduce
the issue again that "first assignment wins" solves in the first place,
which is that it becomes possible to assign a value to a setting, match
on that value and then change the setting later. We acknowledge this by
documenting it in the manual. Also, in some cases, this is exactly what
you want. For example, if you want to use a Fedora rawhide tools tree to
build CentOS 8 images, you have to first match on distribution == centos
and then set Distribution=fedora afterwards for the tools tree preset, so
this actually makes perfect sense in some cases.

While this is technically a compat break, it will only be noticed by users
doing advanced stuff with mkosi, which AFAIK does not exist yet outside of
the systemd repo. In fact even the systemd repo was not broken by this change,
so we should be OK with making it, given the large benefits we get out of it.

This commit also simplifies the interfaces of the parser and matching callbacks
to not take the namespace as an argument anymore, but to simply take the existing
value as an argument instead.