plugin_registry vs. protocol(s): "kludge" -> fixed feature
    
    In fact, commit 8dd14ec29fe829e9edf0102e45966a4a1cc012d7 commented out
    what was intentionally present to prevent RuntimeWarning due to
    importing discovered plugins to the yet non-existent module namespace
    (missing parent) in case the respective registry hadn't been imported
    in advance.  That commit tried to solve a conflict with protol.py,
    which referred to "clufter.plugins" module namespace, but unlike the
    previous registries (formats, filters, commands), "plugins" is not
    backed by a real module (i.e., neither plugins.py nor
    plugins/__init__.py exist at the top level).
    
    Solution is simple, make protocols registry set the namespace as a
    non-value, which is then considered a signal not to attempt any
    such for-the-sake-of-sanity complementary import.
    
    Signed-off-by: Jan Pokorný <jpokorny@redhat.com>