Ignore directories when collecting files to lint (#5775)

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## Summary

<!-- What's the purpose of the change? What does it do, and why? -->

Fixes #5739

## Test Plan

<!-- How was it tested? -->

Manually tested:

```sh
$ tree dir
dir
├── dir.py
│   └── file.py
└── file.py

1 directory, 2 files

$ cargo run -p ruff_cli -- check dir --no-cache
    Finished dev [unoptimized + debuginfo] target(s) in 0.08s
     Running `target/debug/ruff check dir --no-cache`
dir/dir.py/file.py:1:7: F821 Undefined name `a`
dir/file.py:1:7: F821 Undefined name `a`
Found 2 errors.
```

Is a unit test needed?

crates/ruff/src/resolver.rs

@@ -330,9 +330,12 @@ pub fn python_files_in_path(
             }
 
             if result.as_ref().map_or(true, |entry| {
-                if entry.depth() == 0 {
+                // Ignore directories
+                if entry.file_type().map_or(true, |ft| ft.is_dir()) {
+                    false
+                } else if entry.depth() == 0 {
                     // Accept all files that are passed-in directly.
-                    entry.file_type().map_or(false, |ft| ft.is_file())
+                    true
                 } else {
                     // Otherwise, check if the file is included.
                     let path = entry.path();