BUG: String containing list of dependencies was not split. Fixes 0004007

If one was creating an extension that depended on multiple other
extensions and was giving, as specified in the s4ext files, a list of
extensions separated by spaces, Slicer did not split that list when
looking for dependencies upon download, and was trying to find an
extension with all the names.

e.g. If your s4ext file contained the following line:

depends DTIProcess ResampleDTIlogEuclidean

Slicer was trying to resolve the dependencies and was trying
to find the extension named "DTIProcess ResampleDTIlogEuclidean",
instead of finding both extensions separately.

From: Francois Budin <fbudin@email.unc.edu>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24343 3bd1e089-480b-0410-8dfb-8563597acbee