Add dependencies verb

[DaanDeMeyer]

A logical addition to the default tools trees is to allow querying the packages of the tools tree of the current distribution/release so that they can be passed installed all at once. This would look as follows:

sudo dnf install $(mkosi dependencies)

For the implementation, we would do something similar as finalize_default_tools(), except that the only cmdline arguments required will be the distribution, release and architecture, which should be set to the host's distribution, release and architecture respectively. We then parse the mkosi-tools configuration, and print the list of packages on stdout delimited by newlines.