COMP: Fix windows packaging. Fixes 0004131

This commit addresses regression introduced in r24855 (ENH: Add SlicerDMRI
remote module) where the build folder became too long.

This commit introduces the concept of "REMOTE_EXTENSION" allowing to
add source directory to an extension containing multiple modules. Since
the build directory associated with remote extensions is 13 characters
shorter, it should address the windows packaging problem.

Here is an example of path before and after this change:

C:\path\to\Slicer-Release\Slicer-build\Modules\Remote\SlicerDMRI\Modules\Loadable\TractographyDisplay
C:\path\to\Slicer-Release\Slicer-build\E\SlicerDMRI\Modules\Loadable\TractographyDisplay

Notes:

To avoid doing a clean build and ensure incremental build
are successful, developers should update file:

C:\path\to\Slicer-Release\Slicer-build\CMakeCache.txt

and remove variables of the form

"... :INTERNAL= ... Modules\Remote\SlicerDMRI ...".

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24888 3bd1e089-480b-0410-8dfb-8563597acbee