cmGeneratorTarget: collapse paths before querying the fileset cache

Observed with paths with `../` when outside of the source tree. I
attempted to reproduce a test suite case for it but could not make it
happen.

Source/cmGeneratorTarget.cxx

@@ -9435,7 +9435,8 @@ void cmGeneratorTarget::BuildFileSetInfoCache(std::string const& config) const
 
     for (auto const& it : files) {
       for (auto const& filename : it.second) {
-        per_config.FileSetCache[filename] = file_set;
+        auto collapsedFile = cmSystemTools::CollapseFullPath(filename);
+        per_config.FileSetCache[collapsedFile] = file_set;
       }
     }
   }