The Python API for declaring derived settings was confusing to the uninitiated
reader, and also prone to spelling mistakes. This replaces the API with one
that is more readable and more concise, and updates the implementation of
`derive_settings` to properly derive settings declared using the new API.
BREAKING CHANGE: The `derived` and `derived_collection_entry` function are
replaced with the `Derived` class. We do not expect those functions to have
been used outside of edx-platform, but if they are, this commit will cause them
to loudly ImportError.
Note that there should be NO change in behavior to the `derive_settings`
function, which we DO know to be used by some external edx-platform plugins.
Part of: https://github.com/openedx/edx-platform/issues/36215
