When we hit a #include directive that maps to a module import, emit a token
representing the module import rather than making the module immediately
visible. This serves two goals:
 * It avoids making declarations in the module visible prematurely, if we
   walk past the #include during a tentative parse, for instance, and
 * It gives a diagnostic (although, admittedly, not a very nice one) if
   a header with a corresponding module is included anywhere other than
   at the top level.