python: Update build system to ensure dirs.py is created.

python: Update build system to ensure dirs.py is created.

Update build system to ensure dirs.py is created when it is a
dependency for a build target. Also, update setup.py to
check for that dependency.

Fixes: 943c4a325045 ("python: set ovs.dirs variables with build system values")
Signed-off-by: Mark Gray <mark.d.gray@redhat.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>