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/location/SourceLocationFile.h

@@ -2,6 +2,7 @@
 #define SOURCE_LOCATION_FILE_H
 
 #include <map>
+#include <memory>
 #include <ostream>
 #include <set>