Make include dir layout consistent with other KF modules

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

The include visibility layer is named "KFx/Foo", so for the two libs here
is "KF6/Plasma" & "KF6/PlasmaQuick". The duplication with the C++ namespace
as used with the include subdir is confusing, but by chance in the design.