Ref T12845. Config options are "modular", but the modularity is very old, half-implemented, and doesn't use modern patterns.
Half the types are hard-coded, while half the types are semi-modular but in a weird hacky way where you prefix the type with custom:....
The actual API is also weird and requires types to return a lot of array($stuff, $thing, $other_thing, $more_stuff) sorts of tuples.
- Add a new replacement layer which uses modern modularity patterns and overrides the older stuff if available, so we can migrate things one at a time.
- New layer uses a more modern API -- no return array($thing, $other_thing, ...), and more modern building blocks (like AphrontHTTPParameterType).
- New layer allows custom types to be deleted, which will ultimately let us deal with T12845.
Then, convert the 'int' type to use the new layer.