Since we stack using Mac layers as well as Linux layers, both sets of
banner caches are required. As the cache builders are separate at the
moment, we previously ran banner searches without any banners. This was
bad. It inflated memory hugely and killed the buildbot.
We now bomb out on stacking a Linux or Mac layer unless we have banners
against which to match.
Relented on the strict import of direct objects/classes for the typing
module only. Typing module components can be directly imported because
it makes the code really painful to read and write otherwise.
This is still in-line with the python style guide adopted from Google at
http://google.github.io/styleguide/pyguide.html section 2.2.
Mike Auty