Split the modules and include files between weston and libweston

The backends are now installed in lib/libweston-0, and the include
files that will be used by libweston in include/libweston-0. The other
modules and weston-specific include files are kept in the old paths.
A new wet_load_module() is added to load plugins in the old path,
which is not part of libweston, but weston only and defined in main.c.
To allow that to be used by out of tree weston plugins, the function
is declared in a new weston.h, installed in include/weston.

The -0 in the paths is the abi version of libweston, and it will also
be used by the libweston .so. If the abi changes the number will need
to be increased.

Signed-off-by: Giulio Camuffo <giuliocamuffo@gmail.com>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>