move to src/ package layout structure

[jbweston]

Blocked by #238

Currently we use the "ad-hoc" package layout where the "adaptive" package is directly in the repository root.

Such a layout can cause confusion when it comes to testing against the correct version of files, reporting coverage, building documentation etc.

There is a megathread over here: pypa/packaging.python.org#320
that discusses PyPA's official stance on this, but the blog posts I've read have convinced me that the src/ layout has more positives than negatives.

OTOH the current layout works and we have much more interesting things to work on 🤷‍♀️