cmake: Explicitly look up on-disk case of input paths on Windows

KWSys's `CollapseFullPath` no longer looks up the actual case on disk.
Add the lookup explicitly where we need it:

* `ToNormalizedPathOnDisk`
* `file(REAL_PATH)`
* `get_filename_component(... ABSOLUTE)`

Fixes: #20214

Source/cmFileCommand.cxx

@@ -1420,6 +1420,7 @@ bool HandleRealPathCommand(std::vector<std::string> const& args,
     std::string oldPolicyPath =
       cmSystemTools::CollapseFullPath(input, *arguments.BaseDirectory);
     oldPolicyPath = cmSystemTools::GetRealPath(oldPolicyPath);
+    oldPolicyPath = cmSystemTools::GetActualCaseForPath(oldPolicyPath);
     if (warnAbout152) {
       computeNewPath(input, realPath);
       if (oldPolicyPath != realPath) {

Source/cmGetFilenameComponentCommand.cxx

@@ -113,6 +113,7 @@ bool cmGetFilenameComponentCommand(std::vector<std::string> const& args,
       // Resolve symlinks if possible
       result = cmSystemTools::GetRealPath(result);
     }
+    result = cmSystemTools::GetActualCaseForPath(result);
   } else {
     std::string err = "unknown component " + args[2];
     status.SetError(err);

Source/cmSystemTools.cxx

@@ -1664,6 +1664,9 @@ std::string cmSystemTools::ToNormalizedPathOnDisk(std::string p)
 {
   p = cmSystemTools::CollapseFullPath(p);
   cmSystemTools::ConvertToUnixSlashes(p);
+#ifdef _WIN32
+  p = cmSystemTools::GetActualCaseForPathCached(p);
+#endif
   return p;
 }