CMake build: Qt6 build fixes

Set Qt5_VERSION for version checks (e.g. currently still JavaScript
locator filter).

Export special FindQt5.cmake for external plugins and super-repo builds.

Change-Id: I5edb8b0e8bef1a341cd2ff04a13d0a6ce22f6558
Reviewed-by: Cristian Adam <cristian.adam@qt.io>

cmake/FindQt5.cmake

@@ -51,6 +51,7 @@ foreach(comp IN LISTS Qt5_FIND_COMPONENTS)
 endforeach()
 
 set(Qt5_FOUND ${Qt6_FOUND})
+set(Qt5_VERSION ${Qt6_VERSION})
 
 foreach(tool qmake lrelease moc)
   if (TARGET Qt6::${tool} AND NOT TARGET Qt5::${tool})