wayland-scanner can generate header files that only include
wayland-client-core.h using a flag.
Add a core_only option to scan_xml to support this use case.
Thanks to `ModuleInfo`, all modules are just named `foo.py` instead of
`unstable_foo.py`, which simplifies the import method a bit. This also
allows for accurate FeatureNew/FeatureDeprecated use, as we know when
the module was added and if/when it was stabilized.
Mark Bolhuis