Make include dir layout consistent with other KF modules

<include visibility layer> / <C++ namespace> / ForwardCamelCase
<include visibility layer> / <C++ namespace lc> / normal class headers

So as result consumer include statements match the C++ namespace, e.g.
Purpose::Menu -> #include <Purpose/Menu>

The include visibility layer is named "KFx/Foo", so for the two libs here
is KF6/Purpose & KF6/PurposeWidgets.