Support individual codes on `# flake8: noqa` directives (#5618)
## Summary
We now treat `# flake8: noqa: F401` as turning off F401 for the entire
file. (Flake8 treats this as turning off _all rules_ for the entire
file).
This deviates from Flake8, but I think it's a much more user-friendly
deviation than what I introduced in #5571. See
https://github.com/astral-sh/ruff/issues/5617 for an explanation.
Closes https://github.com/astral-sh/ruff/issues/5617.
