...with `std::filesystem` functions

The `stat` defined in `file-system.cc` is not used anywhere now. This
will be removed with the removal of the `STAT` macro defined in the same
file.
`lstat` is still used in some places so we cannot remove the `STAT`
macro.

the `maybeLstat` is removed in favor of the new `maybeSymlinkStat` which
uses the `std::filesystem` library. All the call sites have been updated

... `unix::lstat`

appropriate functions from `std::filesystem` is used in places where
`lstat` was called. Places that require platform specific implementation
are using `unix::lstat` for now.

- The `std::filesystem::status` returns `not_found` and does not throw
  any error.

- `std::filesystem::*` is shortened to `fs::*` for better readability

The result of this function cannot be `std::filesystem::file_status` yet
as we use it to acess `st_mtime`. The equivalent function in
`std::filesystem` to get the last write time does follow symlinks which
is not the behavior we want.

A typo (`"src" -> "dst"`) caught by functional tests is fixed

The removed `nix::maybeLstat` was referenced here. It's replaced with
the new function that replaced it.