Fix FindQt5.cmake, master branch (2019.02.28.)

Unfortunately lcg_wrap_find_module(...) doesn't know how to set up the <FOO>_LIBRARY_DIRS variable when only imported targets are set up by a find-module, and no <FOO>_LIBRARIES variable is available.

So instead of using NO_LIBRARY_DIRS with lcg_wrap_find_module(...), the QT5_LIBRARY_DIRS variable is now set up by hand instead. In a more robust way than was used before, before this issue got introduced.

This problem was unfortunately only seen once I tried to update the master branch with athena!21493 (merged). 😦

Cc-ing @rbianchi on this as well...