SNOW-3010254: Move snowflake.* packages to PEP 420 implicit namespace packages (remove `.pth`-based namespace bootstrap)

Snowflake packages (e.g. snowflake-snowpark-python, snowflake-connector-python) currently rely on setuptools legacy namespace packages, implemented via auto-generated *-nspkg.pth files in site-packages.

Example installed artifact snowflake_snowpark_python-1.39.0-py3.12-nspkg.pth.

This .pth file executes code at interpreter startup to synthesize the snowflake namespace and mutate sys.modules / __path__.

It would be nice to migrate snowflake.* packages to PEP 420 implicit namespace packages, specifically:

1. Remove namespace_packages = ["snowflake"] and any setuptools legacy namespace configuration
2. Ensure no snowflake/__init__.py exists in any distribution, all Snowflake subpackages rely on implicit namespace resolution, and stop shipping *-nspkg.pth files in wheels

This would make Snowflake packages behave like modern namespace ecosystems (e.g. google.*).