demandimport: fix level passed to loader of sub-modules

As the fromlist gives the names of sub-modules, they should be searched in
the parent directory of the package's __init__.py, which is level=1.

I got the following error by rewriting hgweb to use absolute_import, where
the "mercurial" package is referenced as ".." (level=2):

  ValueError: Attempted relative import beyond toplevel package

I know little about the import mechanism, but this change seems correct.
Before this patch, the following code did import the os module with no error:

  from mercurial import demandimport
  demandimport.enable()
  from mercurial import os
  print os.name

mercurial/demandimport.py

@@ -164,7 +164,7 b' def _demandimport(name, globals=None, lo'
         # The name of the module the import statement is located in.
         globalname = globals.get('__name__')
 
-        def processfromitem(mod, attr, **kwargs):
+        def processfromitem(mod, attr):
             """Process an imported symbol in the import statement.
 
             If the symbol doesn't exist in the parent module, it must be a
@@ -172,7 +172,7 b' def _demandimport(name, globals=None, lo'
             """
             symbol = getattr(mod, attr, nothing)
             if symbol is nothing:
-                symbol = _demandmod(attr, mod.__dict__, locals, **kwargs)
+                symbol = _demandmod(attr, mod.__dict__, locals, level=1)
                 setattr(mod, attr, symbol)
 
             # Record the importing module references this symbol so we can
@@ -194,7 +194,7 b' def _demandimport(name, globals=None, lo'
             mod = _hgextimport(_origimport, name, globals, locals, level=level)
 
             for x in fromlist:
-                processfromitem(mod, x, level=level)
+                processfromitem(mod, x)
 
             return mod