mypy struggles to prove this cast correct when there are two or'd
isinstance checks where both types coincidentally have properties of the
same name (but no defined protocol making that explicit). I'm not really
sure why mypy is happy with this in its current state, but it isn't
after a change I'm making.
All our isinstance use is a bit of an anti-pattern anyway, so extract a
method that exposes the data we care about.
The start/end times for tankers aren't actually used, so this could be
simplified even more, but that data _should_ be used.
During package planning we don't care about the details of the flight
plan, just the layout (to check if the layout is threatened and we need
escorts). Splitting these will allow us to reduce the amount of work
that must be done in each loop of the planning phase, potentially
caching attempted flight plans between loops.
Split the oversized file into one per plan type. This also moves the
layout responsibility out of the oversized FlightPlanBuilder and into
each flight plan type file.
