headersclean: process private modules correctly

Change-Id: I51c8ee0db31b35e1d991411fe0b03da6665e9a84
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
make requirement for -private suffix explicit

if a private module was used without the suffix, it would not add any
include paths, but the library would be still added. as long as the
includes were written as <Module/private/Header>, this would not become
visible, as the public modules would add the common include path ...
however, this soon won't be the case for mac frameworks any more. this
change makes the problem visible early on.

Change-Id: I8b1a20313ad736cb49507f07fa623e9aa812f651
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>