COMP: Ensure external module can directly find Slicer

Following commit r22063, extensions bundled using Slicer_EXTENSION_SOURCE_DIRS
didn't have to use the "if(NOT Slicer_SOURCE_DIR)" safeguard.

This commit move inclusion of external modules from the CLI, Loadable
and Scripted sub-directories into a single "Modules/Remote" folder
included after Slicer_DIR so that external module can also directly call
"find_package(Slicer)"

Thanks to Brad Lowekamp for reporting the issue.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22185 3bd1e089-480b-0410-8dfb-8563597acbee