Refactor our generated codebase

[sodic]

Wasp 0.12.0 split our generated code into Framework code (web-app and server) and SDK code (the wasp package).

See #1584 for the initial plan. For story of organizing the SDK, read #1429.

The split is suboptimal, mostly because the SDK contains a lot of the code it doesn't need (it was easier to implement the split without changing too much).

We should go through the modules we generate and reorganize them to minimize the dependencies between them and make them as modular as possible.

This will have an added bonus of speeding up our SDK build step: #1840.