STYLE: Tweak SlicerBlockFindQtAndCheckVersion to display main project name

As it was done in r23926 for the top-level CMakeLists.txt
and SlicerMacroBuildBaseQtLibrary, this commit also updates the
Slicer custom findQt module to display Slicer_MAIN_PROJECT_APPLICATION_NAME
if used from the Slicer source directory or the CMake PROJECT_NAME if used
from outside (case of extension).

Co-authored-by: Sergey Aleshin <4memph@gmail.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24073 3bd1e089-480b-0410-8dfb-8563597acbee