preinstall: simplify OS packages selection

Since a2019c1c (Add a JSON schema describing the packages install
structure, 2024-04-25), we use a custom structure to select which
packages should be installed on a particular host OS.

This has proven too rigid in practice, and the query is pretty
complicated.

Replace this by simply using an array of jinja conditions for the
packages, which should be easier to understand for everyone and more
flexible.

Also remove the associated schema and validation which are no longer
needed.