Rework configuration parsing

Rework configuration parsing

Instead of messing with the internals of argparse, let's implement
proper configuration file parsing.

- Parsing classes and functions are located in a new file config.py
- Configuration settings are split up from CLI settings in a list of
the new ConfigSetting dataclass
- The CLI options for configuration settings now share the same
argparse setting which simply delegates parsing to the corresponding
configuration setting parser.
- We add support for [Match] sections to enable conditionally including
configuration files. Currently Match support is implemented for
Distribution= and Release=.
- Configuration file searching is reworked. In mkosi.conf.d/, we parse
all files ending with ".conf" and directories. If a directory is parsed,
we parse mkosi.conf, mkosi.conf.d/ and all mkosi specific paths in it.
All paths are interpreted relative to the directory that we're parsing.