mkosi: move distro files to mkosi.default.d/

With this change, "mkosi build" will automatically build systemd for the
current distro without any further configuration. If people want to do a
cross-distro build by default, they can still create mkosi.default, but I
assume that this is relatively rare.

If people have symlinked mkosi.default to one of the files in .mkosi/, they'll
need to adjust the symlink.

(Building without configuration would always fail, since systemd has many many
required dependencies. I think it's nicer to do the most commonly expected
thing by default, i.e. rebuild for the current distro.)

Mkosi is nowadays packaged for most distros, so recommend installing of distro
packages as the primary installation mechanism.