base: Switch to use addpylib directive and BB_GLOBAL_PYMODULES

Since bitbake now supports an official method to inject python modules,
switch to it.

Anyone using OE_EXTRA_IMPORTS will need to adjust their code accordingly,
probably switching to their own module namespace.

Also switch to using BB_GLOBAL_PYMODULES to list the global modules
to import.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>