Fix submodule visibility in country modules

[philmas]

Add explicit imports to init.py files for all country modules to make submodules visible via dir() function. This resolves the issue where submodules like stdnum.gb.nhs, stdnum.us.ssn, etc. were not discoverable through introspection despite being importable directly.

Changes:

• Added explicit 'from stdnum.{country} import {module}' statements
• Added '# noqa: F401' comments to suppress unused import warnings
• Ensures all submodules are now visible in dir() output
• Maintains backward compatibility for direct imports

Fixes module discoverability for all country-specific validation modules.

[arthurdejong]

Hi @philmas,

Thanks for your suggestion. I don't think I fully understand the use case behind the change. Introspection works fine with pkgutil.walk_packages() and there is also already a stdnum.util.get_number_modules() function that can help you with that.

For example, to get all the UK number format modules, you can do:

>>> from stdnum.util import get_number_modules
>>> [module.__name__ for module in get_number_modules('stdnum.gb')]
['stdnum.gb.nhs', 'stdnum.gb.sedol', 'stdnum.gb.upn', 'stdnum.gb.utr', 'stdnum.gb.vat']

(by default it will list all the number formats)

[arthurdejong]

Closing this PR given that the functionality already seems to be available. Feel free to re-open.