Guard more system headers by FMT_MODULE (#4006)

* Guard more system headers by `FMT_MODULE`

* Merge FMT_MODULE and FMT_IMPORT_STD

src/os.cc

@@ -12,47 +12,49 @@
 
 #include "fmt/os.h"
 
-#include <climits>
+#ifndef FMT_MODULE
+#  include <climits>
 
-#if FMT_USE_FCNTL
-#  include <sys/stat.h>
-#  include <sys/types.h>
+#  if FMT_USE_FCNTL
+#    include <sys/stat.h>
+#    include <sys/types.h>
 
-#  ifdef _WRS_KERNEL    // VxWorks7 kernel
-#    include <ioLib.h>  // getpagesize
+#    ifdef _WRS_KERNEL    // VxWorks7 kernel
+#      include <ioLib.h>  // getpagesize
+#    endif
+
+#    ifndef _WIN32
+#      include <unistd.h>
+#    else
+#      ifndef WIN32_LEAN_AND_MEAN
+#        define WIN32_LEAN_AND_MEAN
+#      endif
+#      include <io.h>
+
+#      ifndef S_IRUSR
+#        define S_IRUSR _S_IREAD
+#      endif
+#      ifndef S_IWUSR
+#        define S_IWUSR _S_IWRITE
+#      endif
+#      ifndef S_IRGRP
+#        define S_IRGRP 0
+#      endif
+#      ifndef S_IWGRP
+#        define S_IWGRP 0
+#      endif
+#      ifndef S_IROTH
+#        define S_IROTH 0
+#      endif
+#      ifndef S_IWOTH
+#        define S_IWOTH 0
+#      endif
+#    endif  // _WIN32
+#  endif    // FMT_USE_FCNTL
+
+#  ifdef _WIN32
+#    include <windows.h>
 #  endif
-
-#  ifndef _WIN32
-#    include <unistd.h>
-#  else
-#    ifndef WIN32_LEAN_AND_MEAN
-#      define WIN32_LEAN_AND_MEAN
-#    endif
-#    include <io.h>
-
-#    ifndef S_IRUSR
-#      define S_IRUSR _S_IREAD
-#    endif
-#    ifndef S_IWUSR
-#      define S_IWUSR _S_IWRITE
-#    endif
-#    ifndef S_IRGRP
-#      define S_IRGRP 0
-#    endif
-#    ifndef S_IWGRP
-#      define S_IWGRP 0
-#    endif
-#    ifndef S_IROTH
-#      define S_IROTH 0
-#    endif
-#    ifndef S_IWOTH
-#      define S_IWOTH 0
-#    endif
-#  endif  // _WIN32
-#endif    // FMT_USE_FCNTL
-
-#ifdef _WIN32
-#  include <windows.h>
 #endif
 
 namespace {