Dev-only dependencies about flake-parts

[roberth]

flakeModules may suffer from a similar lack of laziness and should perhaps be considered as a "class" or "context" of its own.

Both it and dev-only dependencies move towards a situation where the top level attributes are more controlled, and have to be enumerated somewhere in order to be published on the flake.
It's perhaps worth considering to model these dependency phenomena explicitly within the top level modules somehow. The genericity seems warranted because the internal dependencies may differ between flakes. I'm concerned about the complexity of such a solution though, and about the potential for bad interactions (although that might just be fear talking at this point).
To develop this, it's probably best to start with dev-only dependencies and perhaps later generalize it, if feasible.