Move some FileSystemWatcher usages to FilePath

Change-Id: I72d24ae3f819e429009b0d7e42b21957f07cb00a
Reviewed-by: Marcus Tillmanns <marcus.tillmanns@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/plugins/mesonprojectmanager/mesonbuildsystem.cpp

@@ -185,8 +185,7 @@ void MesonBuildSystem::init()
     m_IntroWatcher.addFile(buildConfiguration()
                                ->buildDirectory()
                                .pathAppended(Constants::MESON_INFO_DIR)
-                               .pathAppended(Constants::MESON_INFO)
-                               .toString(),
+                               .pathAppended(Constants::MESON_INFO),
                            Utils::FileSystemWatcher::WatchModifiedDate);
 }