relax reload_dirs to allow files, and stop adding cwd when reload_dirs is set

[adhami3310]

Summary

Based on the discussion on #1833 the behavior currently done by a uvicorn is intuitive and most likely will lead to bugs by downstream users. It's also the case that watchfiles can watch individual files, as such it's a manufactured constraint by uvicorn is very arbitrary. Some other ASGIs just has reload_paths, that handles files just fine. I didn't change the variable name to not break backwards compatibility, but the former change can be seen as breaking, even if the existing behavior can be considered as a bug.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

I'm not familiar with the larger project, so I will be holding off on documentation and tests until it's verified that the project has interest in the change getting merged.

[stinovlas]

Looks good to me! I bumped into the issue with --reload-dir not behaving like documentation states and this should fix it. Hopefully some maintainer will pick it up soon :-).