lib: fix iteration over schema nodes of a single YANG module

lib: fix iteration over schema nodes of a single YANG module

The only safe way to iterate over all schema nodes of a given YANG
module is by iterating over all schema nodes of all YANG modules
and filter out the nodes that belong to other modules.

The original yang_snodes_iterate_module() code did the following:
1 - Iterate over all top-level schema nodes of the given module;
2 - Iterate over all augmentations of the given module.

While that iteration strategy is more efficient, it does't handle
well more complex YANG hierarchies containing nested augmentations
or self-augmenting modules. Any iteration that isn't done on the
resolved YANG data hierarchy is fragile and prone to errors.

Fixes regression introduced by commit 8a923b48513316b where the
gen_northbound_callbacks tool was generating duplicate callbacks
for certain modules.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>