COMP: Ensure SlicerExecutionModel variable expected by fixup script is set.

This commit fixes a regression introduced by commit r23413 moving
call to find_package where they are needed. By moving the find_package call
from the top-level CMakeLists.txt, variable expected by fixup script were
not set anymore.

It updates SlicerCPack to call find_package and ensure
variable SlicerExecutionModel_LIBRARY_DIRS expected by the fixup script
is set.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23443 3bd1e089-480b-0410-8dfb-8563597acbee