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.

crates/ruff_workspace/src/configuration.rs

@@ -271,7 +271,6 @@ impl Configuration {
                         .chain(lint.extend_per_file_ignores)
                         .collect(),
                 )?,
-
                 fix_safety: FixSafetyTable::from_rule_selectors(
                     &lint.extend_safe_fixes,
                     &lint.extend_unsafe_fixes,
@@ -280,8 +279,9 @@ impl Configuration {
                         require_explicit: false,
                     },
                 ),
-
-                src: self.src.unwrap_or_else(|| vec![project_root.to_path_buf()]),
+                src: self
+                    .src
+                    .unwrap_or_else(|| vec![project_root.to_path_buf(), project_root.join("src")]),
                 explicit_preview_rules: lint.explicit_preview_rules.unwrap_or_default(),
 
                 task_tags: lint