ENH: Explicit specification of ITK COMPONENTS.

SlicerExecutionModel uses find_package(ITK COMPONENTS ...)

Mixing and matching find_package calls with and without COMPONENTS is not very
well supported at the moment.

Use explicit specification of the ITK components and make find_package calls
appropriately local in scope..
Let's hold off on adding this long list of modules.   Let's keep ImageViewer as a "typical" use case and as a test of the usability and backward compatibility of CMake/ITK/Slicer/?.

Consider the future when ITK, for example, adds a new IO method or new module.  I'd rather not have to change the CMakeLists.txt file of every project that uses ITK.   This long list of modules seems to defeat the purpose of having an IO Factory, and this list exposes modularity to developers who really shouldn't have to worry about and adjust to such inner workings of a library when they merely want to link with it.   I'm happy to have ImageViewer and my other code stick with an older version of CMake while these kinks get worked out.