Some refactoring and cleanup (#1806)

* Use clang-tidy `misc-include-cleaner` for IWYU `#include` cleanup

* Use `std::optional<size_t>` instead of `ssize_t`

* Rename some functions in linkdefs.hpp

* Fix header order

src/fix/main.cpp

@@ -1,7 +1,6 @@
 // SPDX-License-Identifier: MIT
 
 #include <sys/stat.h>
-#include <sys/types.h>
 
 #include <errno.h>
 #include <limits.h>