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_java/data/indexer/IndexerCommandJava.h

@@ -4,7 +4,8 @@
 #include <vector>
 
 #include "data/indexer/IndexerCommand.h"
-#include "utility/file/FilePath.h"
+
+class FilePath;
 
 class IndexerCommandJava
 	: public IndexerCommand