`pkgutil.walk_packages` not working when loading libmat2 from `PYTHONPATH`

Hello,

now that setup.py supports installing mat2 into the operating system, the way libmat2 parser modules are included needs to be changed. The relevant part of libmat2/parser_factory.py:

# This loads every parser in a dynamic way
for module_loader, name, ispkg in pkgutil.walk_packages('.libmat2'):
    if not name.startswith('libmat2.'):
        continue
    elif name == 'libmat2.abstract':
        continue
    importlib.import_module(name)

This doesn't work with libmat2 being in PYTHONPATH as pkgutil.walk_packages doesn't search in PYTHONPATH if you give it a path (.libmat2).

The best solution I could find is documented here: https://stackoverflow.com/questions/1707709/list-all-the-modules-that-are-part-of-a-python-package/1707786#1707786