COMP: Ensure NoExtensionSupport component can be specified

Before this commit, the UploadOnly targets were added also if extension
support was explicitly disabled.

This commit ensure that in any given extension, extension support can now
be disabled by either specifying the "NoExtensionSupport" component or by
setting the variable Slicer_DONT_USE_EXTENSION
(or Slicer_USE_NOEXTENSIONSUPPORT) to True.

The message indicating that extension support is disabled has been updated
to be more explicit.

Co-authored-by: Julien Finet <julien.finet@kitware.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22130 3bd1e089-480b-0410-8dfb-8563597acbee