fix #8395, fix #8734: normalize os.tailDir and os.parentDir

lib/pure/os.nim

@@ -330,9 +330,10 @@ proc parentDir*(path: string): string {.
       assert parentDir("./foo") == "."
       assert parentDir("/foo") == ""
 
-  let sepPos = parentDirPos(path)
+  result = normalizePathEnd(path)
+  let sepPos = parentDirPos(result)
   if sepPos >= 0:
-    result = substr(path, 0, sepPos-1)
+    result = substr(result, 0, sepPos-1)
   else:
     result = ""
 
@@ -350,11 +351,10 @@ proc tailDir*(path: string): string {.
     assert tailDir("/usr/local/bin") == "usr/local/bin"
     assert tailDir("usr/local/bin") == "local/bin"
 
-  var q = 1
-  if len(path) >= 1 and path[len(path)-1] in {DirSep, AltSep}: q = 2
-  for i in 0..len(path)-q:
-    if path[i] in {DirSep, AltSep}:
-      return substr(path, i+1)
+  result = normalizePathEnd(path)
+  for i in 0 .. result.high:
+    if result[i] in {DirSep, AltSep}:
+      return substr(result, i+1)
   result = ""
 
 proc isRootDir*(path: string): bool {.