Add PathExists= match

Add PathExists= match

It's also useful to be able to match against stuff that isn't the
current configuration. Let's add a PathExists= match that is satisfied
when the given path exists.

This will be useful in systemd where we conditionally build a kernel if
mkosi.kernel/ exists in the top level repo directory. With this setting,
we can only install the necessary packages to build the kernel if the
mkosi.kernel/ path actually exists.