Declaremods: a few syntactic improvements

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16299 85f007b7-540e-0410-9357-904b9bb8a0f7
Modules and ppvernac, sequel of Enrico's commit 16261

 After some investigation, I see no reason to try to hack
 the nametab in ppvernac, since everything happens there
 at a lower level (constr_expr). So the offending code that
 Enrico protected with a State.with_state_protection is now gone.

 By the way, moved some types from Declaremods to Vernacexpr
 to avoid some dependencies

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16300 85f007b7-540e-0410-9357-904b9bb8a0f7