Consider best approach to avoid circular imports in mixin module

[sadielbartholomew]

We agreed that it would be a good idea to consider if there is a better means to avoid circular imports in cf.mixin than having to hold certain files in a separate mixin2 directory, as implemented as a working but non-ideal solution in #630. See #630 (comment) for details.

This may not require any action i.e. code changes, should we realise the current approach is most suitable, but we should at least think about alternative options and which if any may be best, before closing.