Fix cannot create Windows directory in root (#20311)

* Fix cannot create Windows directory in root

Fixes #20306, a regression bug with `createDir` caused by
`23e0160af2`.

The issue is that, if the path consists only of a drive and a single
directory (e.g. "Y:\nimcache2" in the original issue), then no
directories will be created. This works fine if there are multiple
directories (e.g. "Y:\nimcache2\test").

In the case of "Y:\nimcache2", `omitNext` in `createDir` is `false` on
the last condition in `createDir`. This means that the "nimcache2"
directory will not be created, and no exception will be raised.

Fixed by refactoring to use `parentDirs` iterator instead of iterating
over the string characters. Motivation is reduced code complexity.

Will not test the specific "C:\test" `createDir` case, since there is no
standard Windows drive with write permissions in the root. Creating a
custom drive-mapping to Windows Temp is a non-option. That could mess
up some users running the test.

Added `parentDirs` tests since they are lacking on for POSIX paths.

* Fix `createDir("")` causing error

The change to `createDir` caused `createDir("")` to raise an error,
where it previously didn't. Fixed so `createDir("")` does not fail, and
added test case.

lib/pure/os.nim

@@ -2553,25 +2553,14 @@ proc createDir*(dir: string) {.rtl, extern: "nos$1",
   ## * `copyDir proc`_
   ## * `copyDirWithPermissions proc`_
   ## * `moveDir proc`_
-  var omitNext = false
-  when doslikeFileSystem:
-    var start = 1
-    if isAbsolute(dir):
-      omitNext = true
-      start = dir.splitDrive.drive.len + 1
-  else:
-    let start = 1
-  for i in start.. dir.len-1:
-    if dir[i] in {DirSep, AltSep}:
-      if omitNext:
-        omitNext = false
-      else:
-        discard existsOrCreateDir(substr(dir, 0, i-1))
-
-  # The loop does not create the dir itself if it doesn't end in separator
-  if dir.len > 0 and not omitNext and
-     dir[^1] notin {DirSep, AltSep}:
-    discard existsOrCreateDir(dir)
+  if dir == "":
+    return
+  var omitNext = isAbsolute(dir)
+  for p in parentDirs(dir, fromRoot=true):
+    if omitNext:
+      omitNext = false
+    else:
+      discard existsOrCreateDir(p)
 
 proc copyDir*(source, dest: string) {.rtl, extern: "nos$1",
   tags: [ReadDirEffect, WriteIOEffect, ReadIOEffect], benign, noWeirdTarget.} =