Use a module system for the different integrations

[thufschmitt]

Uses a module-system (inspired by the NixOS one) to make the whole thing a cleaner:

1. Clearly separates the options provided by the modules from their implementations
2. Actually use merging for combining modules rather than abusing contract application (avoids issues like https://github.com/tweag/nickel/issues/1930)
3. Gives a standard way of writing modules so that they can be unified rather than each having a different ad-hoc structure.