QtHelp: Assign security scores to the help module and qhelpgenerator

Mark the code which reads the documentation and collection files and
the search index, all of which may be tampered with, as critical data
parsers, and their headers as containing declarations only.

Mark qhelpgenerator as a build tool, since it only processes the help
project files of the documentation being built.

Task-number: QTBUG-150690
Pick-to: 6.12
Change-Id: I9203af3bdc9b473ef4298201d7dd20b8072fb74b
Reviewed-by: Kai Köhne <kai.koehne@qt.io>