Switch packaging to flit

[maxfischer2781]

flit v3.5 now supports namespace packages: pypa/flit#370

Version 3.5

You can now use Flit to distribute a module or package inside a namespace package (as defined by PEP 420). To do this, specify the import name of the concrete, inner module you are packaging - e.g. name = "sphinxcontrib.foo" - either in the [project] table, or under [tool.flit.module] if you want to use a different name on PyPI (PR #468).

Namespace packaging was the major reason why cobald was not packaged with flit back then. We should strongly investigate whether we can switch now.

[maxfischer2781]

This may need splitting up cobald into individual packages, instead of having a set of "default" packages living inside cobald. Which is probably a good idea anyway...