FindQHelpGenerator: try to find Qt5Help instead of Qt5Core

Summary:
The Qt5Help CMake modules contain the Qt5::qhelpgenerator target, so
look for that instead of Qt5Core.  Otherwise, the Qt5::qhelpgenerator
is available only if Qt5Help was searched previously (which does not
happen often).

Using the Qt5::qhelpgenerator is more reliable than the search for the
qhelpgenerator executable based on the qmake path.

Test Plan:
build some framework with -DBUILD_QCH=ON, and notice that
the right path of qhelpgenerator is found directly with the
Qt5::qhelpgenerator target.

Reviewers: kossebau

Reviewed By: kossebau

Subscribers: kde-frameworks-devel, kde-buildsystem

Tags: #frameworks, #build_system

Differential Revision: https://phabricator.kde.org/D16938