AUTOUIC: Fix internal paths of generated ui_foo.h files

For every file foo.ui we generate a ui_foo.h file in
${target}_autogen/include or ${target}_autogen/include_$<CONFIG> in the
multi-config case.  Even .ui files in subdirectories are handled this
way.  That means, .ui files with the same base name will conflict in a
target.

However, for .ui files in subdirectories we added generated sources with
the nonexistent path ${target}_autogen/include/subdir/ui_foo.h.  This
patch fixes that.

Also, CMake will now yield an error if a target has multiple .ui files
with the same base name.

Fixes #23523

Source/cmQtAutoGenInitializer.cxx

@@ -906,6 +906,7 @@ bool cmQtAutoGenInitializer::InitScanFiles()
   // The reason is that their file names might be discovered from source files
   // at generation time.
   if (this->MocOrUicEnabled()) {
+    std::unordered_set<std::string> addedFiles;
     for (const auto& sf : this->Makefile->GetSourceFiles()) {
       // sf->GetExtension() is only valid after sf->ResolveFullPath() ...
       // Since we're iterating over source files that might be not in the
@@ -947,25 +948,28 @@ bool cmQtAutoGenInitializer::InitScanFiles()
                                                       cmExpandedList(uicOpts));
           }
 
-          auto uiHeaderRelativePath = cmSystemTools::RelativePath(
-            this->LocalGen->GetCurrentSourceDirectory(),
-            cmSystemTools::GetFilenamePath(fullPath));
+          auto uiHeaderFileName = cmStrCat(
+            "ui_"_s, cmSystemTools::GetFilenameWithoutLastExtension(fullPath),
+            ".h"_s);
 
-          // Avoid creating a path containing adjacent slashes
-          if (!uiHeaderRelativePath.empty() &&
-              uiHeaderRelativePath.back() != '/') {
-            uiHeaderRelativePath += '/';
+          // .ui files with the same base name will conflict. Yield an error.
+          {
+            auto insertResult = addedFiles.insert(uiHeaderFileName);
+            if (!insertResult.second) {
+              this->Makefile->IssueMessage(
+                MessageType::FATAL_ERROR,
+                cmStrCat("More than one .ui file with the name "_s,
+                         cmSystemTools::GetFilenameName(fullPath),
+                         " was found in the sources for target "_s,
+                         this->GenTarget->GetName(), "."));
+            }
           }
 
-          auto uiHeaderFilePath = cmStrCat(
-            '/', uiHeaderRelativePath, "ui_"_s,
-            cmSystemTools::GetFilenameWithoutLastExtension(fullPath), ".h"_s);
-
           ConfigString uiHeader;
           std::string uiHeaderGenex;
           this->ConfigFileNamesAndGenex(
             uiHeader, uiHeaderGenex, cmStrCat(this->Dir.Build, "/include"_s),
-            uiHeaderFilePath);
+            cmStrCat("/"_s, uiHeaderFileName));
 
           this->Uic.UiHeaders.emplace_back(
             std::make_pair(uiHeader, uiHeaderGenex));