Avoid top-level imports in `cli.py`

Outputing the help takes about one second on my system:

$ time llm --help > /dev/null
llm --help > /dev/null  0.82s user 0.10s system 99% cpu 0.920 total

That's because cli.py is importing a lot of libraries at the top-level. I propose moving the imports in that file to the functions where they are needed. It may not look asthetic, but it will speed up the call to --help by many orders of magnitudes, and probably somewhat speed up the warm up time when calling subcommands.

Let me know if I can help out with a PR. (It's probably also something that an LLM would be good at.)

Edit: I'm seeing that __init__.py is also affected, and my suggestions probably doesn't apply there ...

Start by reading cli.py and measuring the existing llm --help timing shown in the issue. Move imports that are only needed by specific functions into those functions, while checking the note about init.py separately. Done means help loads substantially faster without changing command behavior.