Item12952: Add Configure/Checkers/Plugins to core

This needs to exist so plugins can populate it.  Otherwise the first
extensions installed creates it as a symlink and ConfigureContrib
can't be installed.

git-svn-id: http://svn.foswiki.org/trunk/core@17863 0b4bb1d4-4e5a-0410-9cc4-b2b747904278