COMP: Change "SuperBuildExtensionTemplate" layout to support addition of modules.

* The new template has been reorganized to easily support the addition of
new modules.

* Folder has been removed from "SuperBuildLoadableExtensionTemplate" to "SuperBuildExtensionTemplate"
to better reflect the role of the template.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21776 3bd1e089-480b-0410-8dfb-8563597acbee