speed up library imports with laziness

[apoorvalal]

Running

uv venv
uv pip install pyfixest tuna
uv run python3 -X importtime -c "import pyfixest" 2> imp.log
tuna imp.log

gives

I almost never use did2s - why am i paying ~2.5 seconds for it on import time? This is why scikit uses lazy imports via _importlib.import_module, which we should emulate

[s3alfisc]

Thanks, we should fix this 👍