Speed up startup time by avoiding CLI import

[natehardison]

Importing the _main.py module in httpx/__init__.py adds 0.087s worth of import time on my machine (MacBook M1 Pro, 32 GB RAM, macOS 15.3.1) running Python 3.13.2.

It's certainly not a huge amount of time in the absolute, but it roughly doubles the amount of time it takes to import httpx, even when you're not intending to run the CLI. Here's an import time graph generated using tuna on the output of python -X importtime -c "import httpx" > import.log

A possible fix is to add httpx/__main__.py that does the import instead. This would also allow python -m httpx as in #3216. I did a quick proof of concept here: master...natehardison:httpx:master

It does change the public API: httpx.main becomes the (more ungainly) httpx.__main__.main.