If find_package(Qt4) is called, then the 'moc' tool used is set to Qt4
even after find_package(Qt5) is called, which creates problems when
both Qt4/Qt5 options are enabled together (i.e. qt, qtgsi, and qt5web).
To solve this problem, searching for Qt4 and Qt5 has been moved down
into the necessary subdirectories. In addition, Qt4 targets were
migrated to use CMake extported targets rather than variables like
QT_LIBRARIES, etc.