Partially reverts: c5c9432ac0
Context: https://github.com/dankamongmen/notcurses/pull/1146
The above commit implemented the much more convenient way of finding and
using a package from CMake, however it broke compatibility with the way
id had been before, which defined various `Notcurses_*` variables.
Revert the `Notcurses` component to the old way and add support for
`Notcurses++` in the same fashion.
Perhaps one day we should think of implementing `find_package` support
by way of a module, which should make it possible to have the cake and
eat the cake. One day.
