Let's create a good rule for which parts of the API are pre-imported

[jonathanhhb]

"It is reasonable and often considered a best practice for Python modules to pre-import public API functions and classes into the init.py file. This approach allows users to access these elements directly from the top-level package (e.g., import mymodule; mymodule.function()), improving usability and intuitiveness, especially for libraries with multiple submodules.

While modern IDEs with autocompletion reduce the need for this practice, pre-importing still offers clear benefits for discoverability and user experience, as users don’t need to navigate submodules or guess where specific functionality resides."

But it does make sense for some API functions to be scoped in a submodule, e.g., the migration math functions. Let's create some rational rule for this so we do as much pre-importing as possible without feeling like it's all very ad hoc.