ProjectExplorer: Pass extra project files as QSet

They are available in some cases as such, and consumed as such.

Change-Id: I9866c7d7bd817fb19a8b11a0efbe583ed55fe393
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/cmakeprojectmanager/fileapireader.cpp

@@ -164,9 +164,9 @@ bool FileApiReader::isParsing() const
     return m_isParsing;
 }
 
-QVector<FilePath> FileApiReader::takeProjectFilesToWatch()
+QSet<FilePath> FileApiReader::projectFilesToWatch() const
 {
-    return QVector<FilePath>::fromList(Utils::toList(m_cmakeFiles));
+    return m_cmakeFiles;
 }
 
 QList<CMakeBuildTarget> FileApiReader::takeBuildTargets(QString &errorMessage){