[[clang-format] Main header search algorithm should consider directories, not only file names

Test case (`test/gui/application.cpp`):

```
#include "test/backend/application.h"
#include "test/gui/application.h"
```

Expected output:

```
#include "test/gui/application.h"
#include "test/backend/application.h"
```

Configuration:

```
BasedOnStyle: LLVM
```

I tried 19.1.1 and `main` (49d48c32e04742ebd05cbbd7dd544b1217b6d9d0).