logic: reduce access to filesystem while indexing

* use canonical filepath cache in cxx indexer
* made constructors of FilePath explicit
* use FilePath at more places instead of string
* forward declares FilePath wherever possible

src/lib/project/SourceGroup.h

@@ -5,10 +5,10 @@
 #include <set>
 #include <vector>
 
-#include "utility/file/FilePath.h"
 #include "settings/LanguageType.h"
 #include "settings/SourceGroupType.h"
 
+class FilePath;
 class IndexerCommand;
 
 class SourceGroup