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/data/parser/Parser.h

@@ -5,8 +5,6 @@
 #include <string>
 #include <vector>
 
-#include "utility/file/FilePath.h"
-
 class ParserClient;
 class TextAccess;