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_cxx/utility/CompilationDatabase.h

@@ -3,7 +3,7 @@
 
 #include <vector>
 
-#include "utility/file/FilePath.h"
+class FilePath;
 
 namespace utility
 {