Detect imports in `src` layouts by default (#12848)
## Summary
Occasionally, we receive bug reports that imports in `src` directories
aren't correctly detected. The root of the problem is that we default to
`src = ["."]`, so users have to set `src = ["src"]` explicitly. This PR
extends the default to cover _both_ of them: `src = [".", "src"]`.
Closes https://github.com/astral-sh/ruff/issues/12454.
## Test Plan
I replicated the structure described in
https://github.com/astral-sh/ruff/issues/12453, and verified that the
imports were considered sorted, but that adding `src = ["."]` showed an
error.
