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.cpp

@@ -1,5 +1,7 @@
 #include "project/SourceGroup.h"
 
+#include "utility/file/FilePath.h"
+
 SourceGroup::~SourceGroup()
 {
 }